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 cooling rack to finish cooling.
Make the Chocolate Ganache:
Add the chocolate chips to a medium sized bowl. Microwave the heavy whipping cream until it almost comes to a boil.
Pour the cream over the chocolate chips, cover the bowl with clear wrap for 3-4 minutes, then whisk until smooth. Set aside to cool.
Make the Peanut Butter Frosting:
While the chocolate ganache cools, make the frosting. Combine the butter, peanut butter and shortening with a mixer until smooth.
Add about half of the powdered sugar and mix until smooth and well combined.
Add 2-3 tablespoons of water or milk to the frosting and mix until well combined.
Add remaining powdered sugar and mix until smooth.
Add additional water or milk, as needed, to get the right consistency, then stir in the crushed butterfinger. Set aside.
Spread a layer of chocolate ganache onto the top of each cupcake.