Preheat the oven to 350 degrees and spray a 9 by 13 inch pan with cooking spray.
In a large bowl, mix together the sugar cookie mix, butter, and egg until fully combined. Press into the bottom of the prepared pan and cook for 15 to 20 minutes or just until the edges of the cookie begin to turn golden brown.
Allow the sugar cookie layer to cool, be sure to wait until it is no longer hot to the touch.
In a medium saucepan heat the sweetened condensed milk and 3 cups of the milk chocolate chips over medium heat, stirring until completely combined. Spread over the cooled sugar cookie layer, making it as even as possible.
In a small saucepan, melt the butter and the remaining one cup chocolate chips over low heat, stirring continuously until fully combined. Pour over the fudge layer and allow to set.
Finally, after the finally layer has set, if you like, melt the white chocolate chips and drizzle over the bars. In the pictures you see above, I have put the white chocolate into a small plastic bag and used it to pipe it over the already cut bars.
Store bars in an airtight container and eat within four days.