Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
Make the Dough: Cream together butter, peanut butter, granulated sugar, and brown sugar until fluffy. Add egg and vanilla, then mix.
Whisk flour, baking soda, and salt in another bowl, then gradually combine with the wet mixture. Fold in Reese’s Pieces and chopped peanut butter cups.
Stuff with Reese’s Cup: Scoop 2 tbsp dough, flatten slightly, and place a mini Reese’s cup in the center. Wrap the dough around it, sealing completely, and roll into a ball.
Bake: Place dough balls on the baking sheet, 2 inches apart. Bake for 10-12 minutes until edges are golden and centers are soft. Optionally, press extra Reese’s Pieces on top.
Cool on the baking sheet for 5 minutes before transferring to a wire rack. Optionally, sprinkle with flaky sea salt and enjoy the molten centers!