Preheat oven to 350°F (176°C) and prepare a cupcake pan with cupcake liners.
In a large mixing bowl, combine sugar, brown sugar, butter, and peanut butter. Using an electric mixer on medium, beat until light in color and fluffy, about 3-4 minutes. Do not skimp on the amount of mixing time.
Add sour cream and vanilla extract and mix until well combined.
Add egg whites in two batches, mixing until well combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
In a separate bowl, whisk together the flour, baking powder, and salt. In a separate small measuring cup, combine the milk and water.
Add half of the dry ingredients to the batter and mix until well combined. Add the milk mixture and mix until well combined. Add remaining dry ingredients and mix until well combined. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
Stir in the crushed butterfingers.
Fill the cupcake liners about halfway. Bake for 18-20 minutes, or until a toothpick inserted comes out with a few crumbs.
Remove cupcakes from oven and allow to cool for 2-3 minutes, then remove to a cooling rack to finish cooling.