When the brownies are completely cooled, prepare the peanut butter frosting.
To prepare the peanut butter frosting, beat together the butter and peanut butter until well combined in the bowl of a stand mixer using the whisk attachment.
On low speed, beat in the powdered sugar and heavy whipping cream until mixture is thick and fluffy and well combined. Thin with more heavy cream if desired to reach the consistency you prefer.
Spread the peanut butter mixture over the brownies.
To make the chocolate frosting, add sugar, butter, and milk to a small saucepan over medium heat and bring to a boil.
Boil for exactly 60 seconds and remove from heat.
Stir in chocolate chips and continue stirring until the chocolate has melted.
Immediately pour the chocolate frosting over the peanut butter layer and gently spread with an offset spatula.
Let the chocolate frosting cool for 5 minutes before sprinkling with chopped Reese's candies.
Let sit for 30 minutes before cutting and serving.