Preheat the oven to 350°F (177°C). Line a 12-count muffin pan with cupcake liners. Line a second pan with 2 liners—this recipe makes about 14 cupcakes. Set aside.

Whisk the cake flour, baking powder, baking soda, and salt together. Set aside.

Using a handheld or stand mixer fitted 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 up the bottom of the bowl with a silicone spatula as needed. Add the egg whites and vanilla. Beat on medium-high speed until combined, then beat in the sour cream. Scrape down the sides and up the bottom of the bowl as needed. 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.

Pour/spoon the batter into the liners—fill only 2/3 full to avoid spilling over the sides. Bake for 19-22 minutes, or until a toothpick inserted in the center comes out clean. Allow cupcakes to cool in the pan for 10 minutes, then remove from the pan to cool completely.

Using a sharp knife, cut a circle/hole into the center of the cupcake to create a little pocket about 1 inch deep. This piece you removed will be the shape of a cone. Set it aside. Place about 1 heaping teaspoon of sprinkles inside the cupcake. Slice off the pointy end of the “cone” piece of cupcake you removed so that it can fit on top of the sprinkle filling. See photos above for visual. Repeat with all cupcakes.

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 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.

Frost cooled cupcakes. I used the Ateco #849 closed star tip.