01 - Preheat oven to 350°F. Line a 12-cup muffin tin with cupcake liners.
02 - Whisk together flour, cocoa powder, granulated sugar, brown sugar, baking soda, baking powder, and salt in a large bowl.
03 - Mix melted butter, eggs, and vanilla in a separate bowl. Add milk and coffee, stirring until combined.
04 - Pour wet ingredients into dry ingredients and mix until just combined.
05 - Divide batter evenly among cupcake liners, filling about 2/3 full. Bake for 16-18 minutes until a toothpick inserted comes out clean.
06 - Cool in pan for 5 minutes, then transfer to a wire rack to cool completely.
07 - Beat peanut butter and softened butter together until smooth. Add powdered sugar and milk, mixing until creamy.
08 - Use a small knife or cupcake corer to remove the center of each cooled cupcake about 1-inch deep. Fill each cavity with peanut butter filling.
09 - Beat butter until creamy. Add powdered sugar, peanut butter, heavy cream, and vanilla. Beat until fluffy, then fold in crushed Butterfinger bars. Add more cream if needed to reach piping consistency.
10 - Frost cupcakes generously. Sprinkle with chopped Butterfinger pieces.