Preheat the oven to 350 degrees F.
In a large bowl, cream together butter, peanut butter, sugar, and brown sugar until smooth.
Add vanilla extract and egg, beating until well combined.
Stir in flour, baking powder, and salt until just incorporated.
Roll dough into balls and place them in well-greased mini muffin pans.
Bake for 8 to 10 minutes.
While baking, unwrap all mini Reese’s peanut butter cups.
Remove cookies from oven and immediately press one mini Reese’s into the center of each cookie so it is flush with the top.
Let the cookies cool in the pans for a few minutes before transferring to cooling racks.