1. Line a mini-muffin tin with paper liners; set aside. Line a baking sheet with parchment paper; set aside.
2. In a medium saucepan, combine peanut butter, butter and brown sugar over medium heat. Stir constantly until mixture is completely melted and starting to bubble a little. Remove from the heat, add the powdered sugar a ¼ cup at a time, stirring until completely combined with the peanut butter mixture. Set aside and let cool.
3. Meanwhile, melt the chocolate chips and shortening together in the microwave on 50% power in 30-second increments, until completely melted, stirring after each increment.