Preheat the oven to 350°F (177°C). Generously grease a 9×13-inch cake pan.

Sift the cake flour, sugar, baking powder, baking soda, and salt in the bowl of a stand mixer. (Or if using a handheld mixer, any large mixing bowl.) With the paddle attachment, beat the ingredients together on low speed for a few seconds to gently combine. Add the butter, vanilla, and 1/2 cup of milk. Mix on medium speed until the dry ingredients are moistened, about 1 minute. Stop the mixer and scrape down the sides and up the bottom of the bowl. The mixture will resemble a thick dough.

Whisk the remaining milk, the sour cream, and eggs together in a medium bowl. With the mixer running on medium speed, add the egg mixture in 3 additions, mixing for about 15 seconds after each addition. Stop the mixer and scrape down the sides and up the bottom of the bowl, then mix for about 15 more seconds until batter is completely combined. Avoid over-mixing. Some small lumps are OK.

Pour and spread batter evenly into prepared pan. Bake for around 32-35 minutes or until the cake is baked through. To test 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 2 minutes. Add confectioners’ sugar, heavy cream, and vanilla extract. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 4 full minutes. Add up to 1/4 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. (I add 1/8 teaspoon salt.) Frosting should be extra fluffy.

Spread frosting in a thick layer on cooled cake. I use and recommend an offset spatula. If desired, use a piping tip to pipe some frosting and/or garnish with sprinkles. Slice and serve.

Cover leftover cake tightly and store in the refrigerator for up to 5 days.