With a standing mixer or hand mixer, whisk egg whites until stiff peaks form. Set aside.

In a separate bowl, sift dry ingredients together (cake flour, salt, baking powder, baking soda). Set aside.

With a hand mixer or standing mixer, cream the unsalted butter until it is light and fluffy, about 1 minute. Add in the granulated sugar, and continue creaming the mixture until light and fluffy, about 1 more minute.

Whisk the buttermilk and vanilla extract together.

Alternately, add the dry ingredients and buttermilk mixture to the butter and sugar mixture, beginning and ending with the dry ingredients, mixing after each addition until just combined.

(I add in 1/3 of the dry ingredients, mix until just combined. Then add in 1/2 of the wet ingredients, mix until combined. And so forth.)

Add in the sour cream, mixing until just combined.

Next, gently fold in the egg whites, folding until the egg whites are fully combined.

Finally, add in the sprinkles (I used 1/2 cup, but you can use up to 2/3 if you prefer more sprinkles), and gently fold the sprinkles into the batter.

Scoop the batter into the prepared cupcake tins, filling about 2/3 of the way full. Tap the cupcake pans on the counter (this helps release any air bubbles).

Bake for 19-22 minutes, or until a cake tester comes out clean and the cupcakes ‘spring’ back when touched in the center.

Allow to cool before frosting.

With a standing mixer or hand mixer, cream the unsalted butter until light and fluffy, and pale yellow in color (about 2-3 minutes).

Add in 3 1/2 cups of powdered sugar. Mix until combined.

Add in 3 Tbsp heavy whipping cream, mixing until combined.

If the frosting seems too thin, add in the remaining 1/2 cup of powdered sugar. If the frosting is too thick, add in the remaining Tbsp of heavy cream.

If using food coloring, add the food coloring to the frosting.

Next, with a tip and piping bag or with a knife, frost the cupcakes and top with sprinkles.