Preheat the oven to 350ºF. Line an 8x8" baking pan with parchment paper, or spray with cooking spray.
In the bowl of a stand mixer or with an electric mixer, combine the brownie mix, egg, cream cheese and butter until it is well mixed. Use a rubber spatula to scrape the sides of the bowl as needed. The batter will be very thick. Spread the chocolate mixture evenly in the prepared pan. 1 Pillsbury family size brownie mix, 1 large egg, 4 ounces cream cheese, 1/4 cup salted butter
Take the frozen Reese's peanut butter cups and press them into the top of the batter in a 4x4 grid pattern. They should be pressed in enough that the top of the Reese's cup is at the same height as the brownie batter. Bake for about 28 minutes. 16 full-size Reese's peanut butter cups
While the brownies are baking, make the peanut butter frosting. Using a stand mixer, blend together peanut butter, butter, powdered sugar and milk for 2-3 minutes or until light and fluffy. 1/2 cup salted butter, 1 cup creamy peanut butter, 3 tablespoons 2% milk, 2 cups powdered sugar
Once the brownies are cool, spread the frosting on top of the brownies.
Place 2 ounces of chocolate in a microwave safe bowl. Heat the chocolate in 15 second intervals. Stir in between each interval until the chocolate is completely melted. Do this slowly so the chocolate doesn't burn. Drizzle the melted chocolate on top of the peanut butter frosting and allow the brownies to set. 2 ounces semi-sweet chocolate
Notes
Make 1/2 a recipe of peanut butter frosting if you want a thinner frosting layer.
Store leftovers in an airtight container at room temperature for up to 2 days. Refrigerate or freeze any leftovers after that.
The calories shown are based on the cake being cut into 16 pieces and all of the frosting used, with 1 serving being 1 frosted brownie. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.