Start by browning the butter. In a medium sauce pan over medium-high heat, melt the butter, stirring often. As the butter cooks it will begin to foam and bubble. Continue cooking until the foam begins to subside and the butter turns an amber brown color and has a nutty aroma. Remove from the heat and pour into a heat proof bowl. Place in the freezer for 15 minutes to cool.
While the butter is cooling, preheat the oven to 350 degrees F. Line an 8x11 baking dish with parchment paper and set aside.
When the browned butter has cooled, start on the dough. In the bowl of a stand mixer, or by hand, begin by beating the browned butter, brown sugar and white sugar. Add the vanilla extract and eggs and beat until smooth.
Add the baking soda, salt and flour. Mix until fully incorporated scrapping down the sides of the bowl as needed. Gently fold in the milk chocolate chips and reese's pieces.
Using your fingers spread half of the dough at the bottom of the baking dish, pressing to form into the bottom and sides of the pan. Dollop the peanut butter over the dough. Top the remaining dough over the top, and gently spread over the peanut butter. It's okay if it's messy or the peanut butter shows.
I wanted these bars thicker and more dense, so I used a smaller 8x11 baking dish, which increased the bake time. For an 8x11 pan these cookie bars bake for 35-40 minutes, or until slightly browned and set. I(f using a larger 9x13 baking dish for thinner cookies, they will only need to bake them for 22-25 minutes).
It's important to allow these cookie bars to cool completely before cutting. While they're cooling, top with candy monster eyes if desired. Cut into bars and ENJOY!