Preheat your oven to 350 degrees Fahrenheit. Spray a 9 by 13 inch baking dish with cooking spray. Set aside.
In a medium bowl whisk together the flour, baking soda, and salt. Set aside.
3 cups all purpose flour, 1 teaspoon baking soda, 1/2 teaspoon salt
In a large bowl, beat together the butter with the granulated sugar and brown sugar until it is light and fluffy, about 3 to 6 minutes.
14 tablespoons unsalted butter, 3/4 cup granulated sugar, 3/4 cup light brown sugar
One at a time, add the eggs and then the vanilla extract. Make sure to scrape down the sides of the bowl as necessary.
2 teaspoons vanilla extract, 2 large eggs
Add all of the flour mixture and either fold it in with a spatula at first or turn on the mixer very low until it begins to combine, then turn the mixer on high and beat until just fully combined. Fold in the M&Ms with a spatula.
1 1/2 cups M&Ms
Press the dough into the prepared baking dish and bake for 20 to 25 minutes or until the edges have just begun to turn golden brown. Allow to cool before slicing.