Step 1

Preheat oven to 350 degrees F. 

Step 2

In a mixing bowl add granulated and brown sugars and melted butter. Beat until well combined. Add eggs and egg yolk, beating after each addition. Add milk and vanilla extract.

Step 3

In a separate bowl mix together the flour, cocoa powder, baking powder, and salt. Add dry ingredients to wet ingredients and mix just until combined. The batter will be thick. 

Step 4

Pour batter into a greased 9x13'' baking pan and spread into an even layer. Bake at 350 degrees for 35-45 minutes or until a toothpick inserted in the center comes out clean. Allow to cool completely.

Step 5

Beat together butter, and peanut butter until smooth. Add powdered sugar and vanilla and beat again until smooth. Add a little splash of milk, beating well until light and fluffy. Add more milk or powdered sugar if needed to make the frosting thicker or thinner. 

Step 6

Smooth frosting over the top of cooled brownies. Refrigerate while you make the chocolate topping.

Step 7

Place chocolate chips and butter in a microwave safe bowl. Microwave in 30 second intervals, stirring in between, until just melted and smooth. Allow to cool for 3-4 minutes. 

Step 8

Spread evenly over the frosted brownies. Add chopped Reese's peanut butter cups on top. Return to fridge to allow the chocolate to set.