Preheat the oven to 350°F/177°C. Line a muffin pan with cupcake liners. Set aside.

In a medium bowl, sift together flour, baking powder, baking soda, and salt. Set aside.

In a large bowl, using a hand-held mixer or stand mixer fitted with the paddle attachment, beat the butter and sugar on medium-high speed until smooth and creamy, 2-3 minutes. Scrape down the sides of the bowl as needed.

Beat in egg and egg whites, one at a time, until combined. Scrape down the sides of the bowl and beat in vanilla until mixed.

With the mixer on low speed and in three parts, alternately add the dry ingredients and the buttermilk to the batter, beginning and ending with the dry ingredients. Beat until just combined. Gently fold in the sprinkles.

Fill cupcake liners ⅔ full and bake for 17-19 minutes, or until a toothpick inserted in the center comes out clean.

Allow cupcakes to cool completely in the pans set on a wire rack before frosting.

In a large bowl, using a hand-held or stand mixer fitted with the paddle attachment, beat the butter until smooth and creamy, 2-3 minutes.

Add the powdered sugar and cocoa powder and mix on low speed until completely incorporated. Mix in the vanilla and enough cream for desired consistency. Increase to medium-high speed and beat for 4-5 minutes until light and fluffy.

Using an offset spatula or disposable pastry bag fitted with a large closed star tip, frost the cupcakes. Top with sprinkles.