• Preheat your oven to 350°F (177°C). Grease your 13x9 cake pan well.

Cake

• Sift together your cake flour, baking soda, baking powder, sugar, and a pinch of salt at low speed in a big mixer bowl. Then, toss in your butter, vanilla extract, and half a cup of milk.
• Crank the electric mixer to medium speed (1 minute) so the mixture looks dough-like.
• Whisk the rest of your milk, eggs, and sour cream in another bowl. Toss this egg mixture into the (still running) mixer in three batches, letting it mix for about 14 seconds after each pour. After the third batch, mix for 15 extra seconds.
• Pour this batter into your prepared pan. Bake for about 32 to 35 minutes until a toothpick poked into the middle comes out mostly clean with very few wet crumbs.
• Let it chill in the cake pan on a wire rack.

Buttercream

• Toss your butter into the mixer and beat (2-3 minutes at medium speed).
• Add your heavy cream, confectioners' sugar, and a splash of vanilla extract. Start at low speed (about 30 seconds), then crank it up to medium-high (4 minutes).
• If the frosting feels too thin, add extra confectioners' sugar (up to a quarter cup). If it's too thick, add another tablespoon of cream. If it tastes too sweet, around ⅛ teaspoons of salt will do.
• Slather the frosting onto your cooled cake, spreading it with a spatula. Optional: Pipe extra frosting with your piping bag or toss on some sprinkles. Enjoy!