Monster cookie bars are a fun treat loaded with peanut butter, oats, chocolate chips, peanut butter chips, and mini m&m's. They bake up perfectly soft, chewy, and thick. A fun treat to make with the kids or great for back-to-school lunches or after school treat.
In a seperate large bowl, or in the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, peanut butter, brown sugar, and sugar until fluffy looking and light in color, 2-3 minutes.
Press the dough into the prepared pan. Bake for 15-18 minutes or until very lightly browned on the edges. The bars will still look pale and slightly underdone in the middle. That's ok. Don't over bake or the bars will be dry!
Let the bars cool in the baking pan for 20-30 minutes. Cut into squares.
If wanted reserve some of the chocolate chips, peanut butter chips, and m&m's to sprinkle on top of the dough before you bake them. Be sure and gently press them down.