Preheat your oven to 350°F (176°C) and line a cupcake pan with paper liners.

In a large mixing bowl, combine the granulated sugar, brown sugar, softened butter, and peanut butter. Using an electric mixer set to medium speed, beat the mixture for 3 to 4 minutes, until it becomes light in color and fluffy. Don’t cut short on this step—it’s key for a soft cupcake texture.

Add the sour cream and vanilla extract, mixing until everything is fully combined.

Next, add the egg whites in two separate additions, beating well after each one. Scrape down the sides of the bowl as needed to make sure all ingredients are thoroughly incorporated.

In a separate bowl, whisk together the flour, baking powder, and salt. In a small measuring cup, stir together the milk and water.

Begin by adding half of the dry ingredients to the batter and mix until just combined. Pour in the milk-water mixture and mix again until well blended. Add the remaining dry ingredients and mix until fully incorporated. Be sure to scrape the bowl down once more to ensure an even mixture.

Gently fold in the crushed Butterfinger candy pieces.

Scoop the batter into the cupcake liners, filling each one about halfway full. Bake for 18–20 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Once baked, remove the cupcakes from the oven and allow them to cool in the pan for 2–3 minutes before transferring them to a wire rack to cool completely.

Place the semi-sweet chocolate chips in a medium heat-safe bowl.

Heat the heavy whipping cream in the microwave until it’s just about to boil.

Pour the hot cream over the chocolate chips and cover the bowl with plastic wrap. Let it sit for 3–4 minutes to allow the chocolate to melt.

Remove the plastic wrap and whisk the mixture until smooth and glossy. Set aside to cool while you make the frosting.

While the ganache is cooling, prepare the frosting. In a large bowl, use a mixer to blend the butter, peanut butter, and shortening together until smooth.

Add half of the powdered sugar and beat until fully incorporated.

Mix in 2 to 3 tablespoons of water or milk to loosen the frosting slightly.

Add the remaining powdered sugar and beat again until smooth and fluffy.

If needed, add more water or milk, one tablespoon at a time, to reach your desired piping consistency. Stir in the crushed Butterfinger pieces, then set the frosting aside.

Once the cupcakes are cool and the ganache has set to a spreadable consistency, spoon or spread a thin layer of ganache over the top of each cupcake.

Fill a piping bag fitted with a large round tip with the prepared peanut butter frosting. Pipe a swirl of frosting on top of each ganache-covered cupcake.

Finish each cupcake by sprinkling with additional crushed Butterfinger candy and, if desired, top with a small piece of Butterfinger for decoration.