Using a hand mixer on medium speed, beat brown and white sugars, peanut butter, shortening, butter, and egg in a large bowl.
Whisk together flour, salt, baking soda, and baking powder together in a medium-sized bowl.
Turn the mixer to low speed and add the flour mixture. Mix until combined and dough is formed.
Cover peanut butter cookie dough and chill dough for about 2 hours or until firm.
Preheat oven to 375°F and spray a mini muffin pan with nonstick cooking spray.
Roll the dough into balls about an inch to an inch and a half in diameter. That equals about one tablespoons worth.
Place each cookie dough ball into the mini muffin tin.
Bake cookies for 12 minutes or until light golden brown. While the cookies are baking, unwrap mini Reese’s peanut butter cups.
Press mini peanut butter cups into the center of each warm cookie. Cool and remove from muffin tin and place cookies on a wire cooling rack.