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Monster Cookie Bars

4.67 from 6 votes

posted: 04/04/20

This post may contain affiliate links. Please read my disclosure policy
This post may contain affiliate links. Please read my disclosure policy

Monster Cookie Bars are so easy to make, and full of delicious flavor. Such a fun treat to bake with the kiddos!

close up of four monster cookie bars with the a bite missing out of the top one

We are incredibly lucky that our kids attend a school district with award winning teachers. They have made virtual learning easier and this transition into home schooling smoother.

Daily we are to do a science project with our 3rd grader, and one of the suggestions was baking. So we made Monster Cookie Bars! It was great to be able to teach the science of baking, like how the brown sugar works with the baking soda to form a reaction.


These are the bar version of my Monster Cookies.

Cookie Bars are a GREAT recipe for a first time baker. You can get things a little bit wrong (too much flour, not soft enough butter, etc) and still end up with a great dessert.

a stack of four monster cookie bar recipe on a black rimmed tray


The reason for the name Monster Cookies is because we are taking every kind of cookie and mashing them together here! These cookies contain:

  • M&Ms
  • Oatmeal
  • Chocolate Chips
  • Peanut Butter

They are a hodge podge wonder dessert!


Though cookie bars are very forgiving, I think they still present a great opportunity to teach our kiddos baking basics.


Resist the urge to scoop the measuring cup right into the bag of flour to get it out. Some bakers estimate that you can get up to 25% more flour that way. A great way to ruin a cookie recipe.

Instead, scoop the flour out with a spoon into the measuring cup, and then level it off with a flat edge.


Creaming butter and sugar together is a common step in cookie recipes. The reason for it is that we are creating air pockets in the butter with sugar. It results in light fluffy cookies.

Additionally, if you successfully cream your butter and sugar for long enough you can actually get more cookies out of your dough recipe. (You can check out this article from Serious Eats to learn more.)


To get baked goods that rise, you need baking soda and baking powder that are fresh and will still create a reaction.

This is how you test each:

  • Baking Soda: Drop it in a little vinegar and make sure it bubbles right away.
  • Baking Powder: Drop it in some water and make sure it bubbles right away.


top down look at four monster cookie bars with chocolate chips and M&Ms in them.


Allow these bars to cool completely before cutting them.

These make very very rich cookie bars. I highly suggest that you cut them to make at least 24 bars.


After allowing the bars to cool completely, store them in an airtight container for up to 1 week.

Alternatively, freeze on a parchment lined baking sheet for 1 hour and then transfer to an airtight container. Freeze for up to three months.

PRO TIP: Nathan likes eating these right from the freezer!

front on view of four monster cooke bars stacked with a milk glass with striped straws in the background


If you can’t get over cookie bar recipes, make these with the kids!

If you love these cookie bars or make others, please leave me a comment and let me know what you think!

close up of four monster cookie bars with the a bite missing out of the top one
4.67 from 6 votes

Monster Cookie Bars

Serves: 24 bars
(tap # to scale)
Prep: 20 minutes
Cook: 27 minutes
Total: 47 minutes
Monster Cookie Bars are so easy to make, and full of delicious flavor. Such a fun treat to bake with the kiddos!


  • 1 cup all purpose flour see note 1
  • 1/2 teaspoon baking soda
  • 1/2 teaspoon baking powder
  • 1/4 teaspoon salt
  • 1 cup unsalted butter room temperature
  • 1 cup brown sugar packed
  • 1/2 cup granulated sugar
  • 1 1/2 cups creamy peanut butter
  • 2 large eggs room temperature
  • 2 teaspoons vanilla extract
  • 1 cup chocolate chips (I used milk chocolate)
  • 1 cup M&Ms
  • 1 cup quick cooking or old fashioned


  • Preheat your oven to 350 degrees. Spray a 9 by 13 inch pan with cooking spray and set aside.
  • In a medium sized bowl, mix together the flour, baking soda, baking powder, and salt until fully combined. Set aside.
  • In a large bowl, beat together the butter, brown sugar and granulated sugar, until light and fluffy. About three minutes. Then proceed to beat in the peanut butter until fully combined.
  • Beat the eggs into the butter/sugar/peanut butter mixture, one at a time. Finally, beat in the vanilla.  
  • Add in the flour mixture all at once (see note 2) mixing it in on a very low speed until you have no risk of flying flour and then up the speed just a little and mix in until the flour has just disappeared.
  • Finally, stir in the chocolate chips, M&Ms, and oats.
  • Dish the batter into the baking pan. It will be very sticky. Spray a spatula with cooking spray and use that to spread out the dough.
  • Bake for 25 to 27 minutes or until the edges of the bars are golden brown. Allow to cool completely before cutting into 24 equal squares.
  • Store in an airtight container for up to 1 week. See notes in the post about freezing.


  1. To measure the flour, spoon it into the measuring cup and level off the top, rather than scooping it right from the flour container.
  2. Most cookie recipes suggest that you mix in the flour a little at a time so that the flour does not fly everywhere and it suggest that you not over-mix. I recently read that you should add it all at once, mixing in slowly at first so it doesn’t go everywhere, and then just mixing until the flour disappears. This lowers the risk of over stimulating the gluten which can lead to a dense cookie.
Author: Lisa Longley
Course: Dessert
Cuisine: American
close up of four monster cookie bars with the a bite missing out of the top one

did you make this

Monster Cookie Bars

I’d love to see what you made! Tag me @lisasimplejoy and hashtag it #simplejoyrecipes!

Hi! I'm Lisa Longley, and I am committed to giving you simple dinner ideas and recipes that are easy to make; recipes that will fill your home with joy. I am the owner and author of SimpleJoy.com and I'm so glad that you are here.

Reader Interactions


    Have a question? Use the form below to submit your question or comment. I love hearing from you & seeing what you made!

    Rate This Recipe:

  1. Alissa K. says

    Just the pictures of these look so delicious, cant wait to try! 👾🍪

    • Lisa Longley says

      I hope you love them, Alissa!

  2. Brenda says

    3 stars
    Way too much pb, will cut back on it next time.

  3. Heather says

    You’re missing the oats in the instructions and it didn’t seem like enough..1 cup?? Now i have to go find another recipe to fix this

    • Lisa Longley says

      The oats are added in step 6. I don’t know what to tell you about them not being enough – 1 cup was the perfect amount for this recipe.

  4. Anne spencer says

    Hiya what is quick cooking or old fashion mean in your recipe for monster cookies

    • Lisa Longley says

      That refers to the type of oats I use! They are sold both as old fashioned and quick cooking, and either will work in this recipe.

  5. Amanda says

    Made a double recipe, I reduced the peanut butter slightly and subbed half of the butter for coconut oil. They look fantastic

    Also I mixed all my dry ingredients together and had no issue with folding in

  6. Joan says

    Sounds delicious, but my grandson is very allergic to peanut butter… do you have a similar recipe without the PB?

    • Lisa Longley says

      You might enjoy my M&M Cookie Bars. Otherwise you could replace it with sunflower butter and that would work just fine here.

  7. Bobbi says

    Love baking, but have one question on this recipe. If I use salted butter, can I just omit the salt or cut it back? Or does unsalted butter react different in baking?

    • Lisa Longley says

      It doesn’t react differently, its just a way to control the salt in the recipe. So if you use salted, I would omit the salt completely the first time you make it.

  8. Trisha Monroe says

    These look delicious. I love the thought of all the different ingredients added. Yes, definitely like several cookie flavors all rolled into one. I’m planning on making these in 3 days.
    A side note, in step 8, you write “Allow to cut compltely….”, It should read “Allow to cool completely….”.
    I apologize for sharing this, I tend notice things and this just stood out. (I get it from my mom.) I mean no disrespect.

    • Lisa Longley says

      Thank you!

  9. Lisa says

    I was excited to make these. However, I’m having trouble getting them done in the center. They are still soupy after about 40 minutes of baking. Probably won’t make them again.

    • Lisa Longley says

      Hey Lisa! I haven’t had that problem with these at all. Is it at all possible that your oven temperature is off? My other thought is the baking dish. A metal baking dish can really help with conduction. This is the one I love: https://amzn.to/3y4D47o

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5 Secrets to Stress Free Dinners



5 Secrets to Stress Free Dinners