1. Preheat oven to 350 F and line a 12-cup cupcake tin with paper liners.
2. In a large mixing bowl, whisk together the flour, baking powder and salt. In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a handheld mixer, cream the butter on high until creamy, about 1 minute. Add the sugar, and beat on high for an additional 2 minutes, scraping down the sides of the bowl as required. Add the egg whites, one at a time, continuing to beat on medium-high speed. Add the vanilla, then scrape down the sides of the bowl and beat together until thoroughly mixed.
3. With the mixer on low, add the dry ingredients slowly until incorporated. Pour in the buttermilk, with the mixer still on low. Mix for one minute until just combined. Using a spatula, gently fold in the sprinkles.
4. Distribute the batter evenly among the cupcake pan, filling until about 3/4 full. Bake for 20-22 minutes or until a toothpick comes out clean.
5. Allow the cupcakes to col completely before frosting.

Frosting

1. In the bowl of a stand mixer, or in a large bowl using a handheld mixer, beat the butter on medium-high speed until creamy. Add the powdered sugar, vanilla, and cream, beating until all the ingredients are incorporated and frosting is fluffy. Add more cream if frosting is too thick, and more powdered sugar if it is too thin. Frost the cupcakes!

Cupcakes will keep at room temperature, in an airtight container for 4-5 days. Frosted cupcakes will also freeze well in an airtight container; thaw overnight in the fridge before consuming.