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.
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.
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.
Peanut Butter Frosting:
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.
Smooth frosting over the top of cooled brownies. Refrigerate while you make the chocolate topping.
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.
Spread evenly over the frosted brownies. Add chopped Reese's peanut butter cups on top. Return to fridge to allow the chocolate to set.
Make ahead Instructions: These brownies taste great made 1-2 days in advance, stored covered in the refrigerator. The peanut butter frosting can be made 1-2 weeks in advance. Bring to room temperature before spreading on brownies. Freezing Instructions: Baked and cooled brownies can be frozen for up to 3 months. Thaw overnight in the fridge, or on the counter, before adding frosting and glaze.