Yellow cake with chocolate buttercream frosting

RECIPE

For the cakes

• 1.5 sticks (3/4 cup, 170g) butter, softened
• 1.5 cups (300g) granulated sugar
• 2 whole eggs
• 2 egg yolks
• 1.5 cups (350mL) buttermilk (could be sour cream or crème fraîche diluted to a thick milk texture)
• 2-3 teaspoons vanilla
• 2.5 cups (300g) cake flour (can be 7 parts AP flour + 1 part starch)
• 1/2 teaspoon baking soda
• 2 teaspoons baking powder
• 2-3g (1/2 teaspoon of Morton kosher) salt (double that if using unsalted butter)

For the frosting

• 2 sticks (1 cup, 225g) butter, softened
• 1/2-1 cup (50-100g) cocoa powder, depending on how chocolatey you want it (I used dutch cocoa)
• 2.5 cups (300g) powdered sugar
• 1/2 cup (120mL) corn syrup (or any syrup)
• 1 heaped tablespoon meringue powder (highly optional, can be found in the U.S. at craft stores)
• vanilla
• milk/buttermilk/water
• sprinkles, etc (if you're into that)

Prepare two 8-inch (20cm) diameter cake pans by greasing them and then lining the bottoms with parchment rounds (watch the video to see how to make those).
For the cake batter, put the softened butter in a big mixing bowl and whip it as fluffy as possible. Put in the sugar and whip some more until as fluffy as possible. Gradually beat in the eggs and yolks, buttermilk and vanilla. Beat in the dry ingredients, adding enough flour to get a pudding-like texture in the final batter.
Split the batter evenly into the two pans and bake at 350ºF/180ºC until a toothpick or knife to the center comes out clean — mine took 30 min. Let cool and solidify until you can invert them out of their pans and peel off the paper. Before frosting, let them cool some more until they are no warmer than room temperature — you can speed this on a cooling rack in the fridge.
To make the frosting, whip the butter until fluffy and then beat in the powdered sugar, followed by the cocoa powder. Beat in the corn syrup and meringue powder together. Beat in vanilla and additional salt (if you used unsalted butter) to taste, and just a little buttermilk or water to get it to a spreadable consistency.
Use a bread knife to shave the domes off of the cooled cakes and make them flat. Put one take on a serving plate, top with almost half your frosting, and use a long knife or icing spatula to spread to an even thin layer, allowing the excess to fall over the sides. Put the other cake on top, dump on all the remaining frosting and repeat the process. Spread icing around the sides, clean up the rim of the plate with a damp towel and top with sprinkles, if you're into that. I prefer the firmer texture of this icing when the cake is refrigerated.