Lightly grease a muffin pan with nonstick cooking spray, blotting any pools of oil with a paper towel. Set aside.
In a large stainless steel mixing bowl set over a pot of barely simmering water (making sure that the bottom of the bowl does not touch the water), melt chocolate until smooth. Stir in butter until fully incorporated.
Using a 2 tablespoon measure, pour the melted chocolate into the bottom of each mold. Place into the refrigerator to set for 10 minutes.
Remove the muffin pan from the refrigerator. Add 1 teaspoon of peanut butter into the center of each mold. Top with 1 teaspoon of Butterfingers. Top with 2 tablespoons of melted chocolate, making sure to cover the filling. Use the back of a spoon to smooth the surface if necessary. Add a sprinkle of Butterfingers on top.
Refrigerate for 30 minutes until set.
Use a small offset spatula to remove the chocolate cups from the mold. Store at room temperature.