Preheat oven to 350°F. Line a 12-count muffin tin with paper liners.
Heat the peanut butter in a small microwave-safe bowl for 30-45 seconds, or until it is easily pourable. Set aside.
⅓ cup creamy peanut butter
In a large bowl, whisk together the milk, vegetable oil, eggs, and vanilla.
½ cup whole milk, ⅓ cup vegetable oil, 2 large eggs, 1 teaspoon pure vanilla extract
Add the peanut butter and brown sugar and whisk to combine.
1 cup light brown sugar
Slowly add the flour and baking powder, mixing just until the dry ingredients are fully incorporated. You don’t want to overmix.
1½ cups all-purpose flour, 1 teaspoon baking powder
Add the chopped peanuts and Butterfingers and gently fold them into the cake batter.
¼ cup finely chopped Butterfinger candy, ¼ cup finely chopped peanuts
Divide the batter between the tins, filling each one ¾ of the way full.
Bake for 15-17 minutes, or until a toothpick inserted into the center comes out dry (it may be sticky from the butterfingers, but it shouldn’t be wet).
Remove the cupcakes from the pan and allow them to cool on a wire rack before frosting.