Preheat oven to 350°F (177°C). Grease a 9×2-inch round cake pan, line with a parchment paper round, then grease the parchment paper. Parchment paper helps the cakes seamlessly release from the pans. (If it’s helpful, see this parchment paper rounds for cakes video & post.)

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

Using a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter on high speed until smooth and creamy, about 1 minute. Add the sugar and beat on high speed until creamed together, about 2 minutes. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. On high speed, beat in the egg whites until combined, then beat in the sour cream and vanilla extract. Scrape down the sides and up the bottom of the bowl as needed. Pour the dry ingredients into the wet ingredients. Turn the mixer to low speed and as the mixer runs, slowly pour in the milk. Beat on low speed just until all of the ingredients are combined. Do not over-mix. 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. Gently fold in the sprinkles.

Pour batter evenly into prepared cake pan.

Bake for around 27-31 minutes or until the cake is baked through. After about 18 minutes, tent the cake with aluminum foil to prevent the top and edges from over-browning. To test the cake for doneness, insert a toothpick into the center of the cake. If it comes out clean, it’s done.

Allow cake to cool completely in the pan set on a wire rack. The cake must be completely cool before frosting.

With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 1 minute. Add the confectioners’ sugar, heavy cream, and vanilla extract. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 2 full minutes. Add up to 1/4 cup (30g) more confectioners’ sugar if frosting is too thin or a splash of heavy cream if frosting is too thick. Add a pinch of salt if frosting is too sweet. (I add a small pinch.)

Spread frosting all over the top. I always use an icing spatula. Top with sprinkles, if desired.

Cover leftover cake tightly and store at room temperature for up to 1 day or in the refrigerator for up to 5 days.