Preheat oven to 350 degrees F. Line a cupcake tin with paper liners. Set aside.

For the cake, combine cake flour, baking powder, and kosher salt in a bowl. Set aside.

In a mixing bowl, beat butter and sugar for 2 minutes on medium, scraping down the sides of the bowl as needed. Beat in egg whites and vanilla extract.

Alternate additions of dry ingredients with buttermilk, mixing after each addition (about 3 additions of each). Once the last of the buttermilk is added, beat for one full minute.

Fold in the sprinkles gently.

Fill cupcake liners two-thirds full with batter and bake for about 21-23 minutes, or until a toothpick inserted into the center comes out clean. Remove and cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before frosting.

In a mixing bowl, beat softened butter until creamy.

Add cocoa powder and beat until combined.

Gradually add powdered sugar, alternating with milk, beating until smooth and creamy. Add vanilla extract and beat until well combined.

Once cupcakes are completely cooled, frost them with the chocolate frosting. You can pipe it on or spread with a spatula.