Mix peanut butter, sugar, and egg.
Put the mixture in the fridge for 20 minutes to firm it up.
Line a baking sheet with parchment paper.
Make peanut butter balls about 1 Tbsp each.
Place the balls on the parchment paper.
Bake in the oven at 350°F for about 15 minutes.
Top with Reese's cups.
Let the cookies sit and cool for 20 minutes.
