Preheat the oven to 350°F. Line a muffin pan with 8 cupcake liners. Set aside.

Make the cupcakes: In a medium mixing bowl, whisk together the flour, cornstarch, baking powder, baking soda and salt. Set aside.

In a large mixing bowl or a bowl of a stand mixer with a paddle attachment, beat the butter on high sped until light creamy. Add in the sugar and beat for 1-2 minutes until light and fluffy, scraping the bottom and sides of the bowl as needed. Add the egg whites and vanilla then beat on high for 1 minute until combined. Then, beat in the sour cream— scrape the bottom and sides of the bowl to make sure it's well combined.

With the mixer on low, add in the dry ingredients ¼ cup at a time. Then, keeping the mixer on low add the milk and beat on low until just combined. Do not overmix.

Fill each cupcake liner about ⅔ of the way full with batter. Bake for 20-21 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs. Allow the cupcakes to cook completely.

Make the frosting: With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2-3 minutes, scraping the bottom and sides of the bowl as needed. Add 4 and ½ cups powdered sugar, in ¼ cup increments— this helps reduce the mess. Then, add the heavy cream and vanilla extract. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 2-3 minutes. Add up to ½ cup more powdere’ sugar if the frosting is too thin or another Tablespoon of cream if frosting is too thick. Add a pinch of salt if frosting is too sweet. I normally end up adding ⅛-1/4 teaspoon.

Fill the cupcakes: Using a sharp knife, cut a hole into the center of the cupcake about 1-inch deep. The piece that you remove should be roughly shaped like a cone. Set aside. Place ½-1 teaspoons of sprinkles inside the cupcake. Add the cupcake piece back to the top and slightly press down. Repeat with all cupcakes

Frost the cooled and filled cupcakes as desired.