In a large bowl, beat together the brown sugar, granulated sugar, peanut butter, shortening, softened butter, and egg until well combined.

In a medium bowl, whisk together the all-purpose flour, salt, baking soda, and baking powder.

Lower the mixer speed and gradually incorporate the flour mixture into the wet ingredients until a dough forms.

Cover the dough with plastic wrap and chill it in the refrigerator for about 2 hours or until firm.

Preheat your oven to 375°F (190°C) and spray a mini muffin pan with non-stick cooking spray.

Roll the dough into balls, roughly one tablespoon in size, and press them into the muffin tin.

Bake for approximately 12 minutes or until they are light golden brown.

Unwrap the mini peanut butter cups, and gently press one into each warm cookie, allowing it to melt slightly.

After cooling in the muffin tin briefly, transfer the cookies to a wire rack to cool completely.