In a large bowl, combine brownie mix, water, oil, and egg.
Lightly grease a mini muffin pan.
Fill each muffin cup 2/3-full with brownie batter.
Bake for 10-12 minutes, or until edges appear crispy, and centers are mostly set.
Remove from heat, and immediately press a mini Reese’s cup in the center of each brownie.
Use a small knife to carefully lift out the brownie cups to a wire rack. They will still be warm, and they will be soft, but you don’t want them to cool in the pan, or they’ll stick and will become difficult to remove. The Reese’s cups will start to melt a bit, but this is fine – they will get covered with buttercream, so they don’t need to look perfect.
Cool brownie cups completely.
For the buttercream, in a large mixing bowl, combine butter, peanut butter, vanilla, powdered sugar, and heavy cream.
Beat until light and fluffy, about 5 minutes.
Pipe the buttercream frosting onto the top of each brownie cup.