01 - Preheat oven to 350°F. Line a 12-cup muffin tin with cupcake liners.
02 - In a large bowl, whisk together flour, cocoa powder, granulated sugar, brown sugar, baking soda, baking powder, and salt until well blended.
03 - In a separate bowl, whisk melted butter, eggs, and vanilla until smooth. Add milk and coffee, stirring until fully incorporated.
04 - Pour wet ingredients into dry ingredients. Mix until just combined, being careful not to overmix.
05 - Divide batter evenly among cupcake liners, filling each about 2/3 full.
06 - Bake for 16 to 18 minutes, or until a toothpick inserted into the center comes out clean. Let 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 and creamy. Add powdered sugar and milk, mixing until fluffy and well combined.
08 - Once cupcakes are completely cool, use a small knife or cupcake corer to remove the center of each 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 extract. Beat until light and fluffy, then fold in crushed Butterfinger bars. Add additional cream if needed to reach piping consistency.
10 - Generously pipe or spread frosting over filled cupcakes. Sprinkle with coarsely chopped Butterfinger pieces before serving.