1 cup unsalted butter, softened to room temperature
4 - 5 cups confectioners' sugar
1/4 cup heavy cream or whole milk
2 teaspoons pure vanilla extract
salt, to taste
Making the cupcakes
Preheat the oven to 350°F (177°C).
Whisk the cake flour, baking powder, baking soda, and salt together. Set aside.
Using a handheld or stand mixer with a paddle attachment, beat the butter on high speed until smooth and creamy - about 1 minute.
Add the sugar and beat on high speed for 2 minutes until creamed together. Scrape down the sides and bottom of the bowl with a rubber spatula as needed.
Add the egg whites and vanilla. Beat on medium-high speed until combined, then beat in the sour cream.
With the mixer on low speed, add the dry ingredients until just incorporated. With the mixer still running on low, slowly pour in the milk until combined. Do not overmix.
You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl. The batter will be slightly thick.
Line a muffin pan with cupcake liners (recipe makes about 14 cupcakes). Fill liners only 2/3 full with batter. Bake for 19-22 minutes.
Filling the cupcakes
Let Cupcakes cool. Then cut a circle/hole into the center of the cupcake to create a cone shaped pocket about 1 inch deep. Remove this piece and cut some of the pointed end, just enough to leave space for sprinkle filling.
Place about 1 heaping teaspoon of sprinkles inside the cupcake. Then replace the cupcake's middle.
Making the frosting
With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy - about 2 minutes.
Add 4 and 1/2 cups confectioners' sugar, the heavy cream, and vanilla extract, and protein powder with the mixer running on low.
Increase to high speed and beat for 3 full minutes.
Add up to 1/2 cup more confectioners' sugar if frosting is too thin or another Tablespoon of cream if frosting is too thick. Add a pinch of salt if frosting is too sweet.