For the vanilla cake:

1. Heat the oven to 350 degrees. Coat two 9-inch round cake pan with nonstick spray and line the bottom with parchment paper.
2. Beat the butter and sugar together in a large blown with an electric mixer until fluffy. Beat in the eggs, yolks, and vanilla, until combined, scrape down the bowl, and then sprinkle the baking powder, baking soda, and salt over the batter and beat until very well combined. Add the butter milk and mix to combine; the batter may look curdly, but this isn’t a problem. Add the cornstarch and flour, and mix until the flour just disappears.
3. Evenly divide the batter into the prepared pans (weighing or measuring the batter is super helpful for accuracy) and smooth the tops. Bake for 25 to 27 minutes, until a toothpick inserted into the center of the cake comes out batter-free. Let cool in the pan on a cooling rack for 10 minutes, then run a knife around the cake to ensure its loose, flip it out onto a cooling rack, carefully remove the parchment, and then flip back right side up onto another. Let cool completely before frosting. You can hasten this process along in your freezer, where it shouldn’t take more than 15 minutes to fully cool.

For the cinnamon chocolate cake: 

1. Heat the oven to 350 degrees. Coat two 9-inch round cake pan with nonstick spray and line the bottom with parchment paper.
2. Beat the butter and sugar together in a large blown with an electric mixer until fluffy. Beat in the eggs, yolks, and vanilla, until combined, scrape down the bowl, and then sprinkle the baking powder, baking soda, and salt over the batter and beat until very well combined. Add the butter milk and mix to combine; the batter may look curdly, but this isn’t a problem. Add the cocoa, cinnamon, and flour, and mix until the flour just disappears.
3. Evenly divide the batter into the prepared pans (weighing or measuring the batter is super helpful for accuracy) and smooth the tops. Bake for 20 to 30 minutes, until a toothpick inserted into the center of the cake comes out batter-free. Let cool in the pan on a cooling rack for 10 minutes, then run a knife around the cake to ensure its loose, flip it out onto a cooling rack, carefully remove the parchment, and then flip back right side up onto another. Let cool completely before frosting. You can hasten this process along in your freezer, where it shouldn’t take more than 15 minutes to fully cool.

For the buttercream:

In a large bowl using a hand-held mixer or stand mixer fitted with a whisk or paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. Add confectioners’ sugar, milk, vanilla extract, and salt with the mixer running on low. Increase to high speed and beat for 2 minutes. Add more confectioners’ sugar if frosting is too thin, more milk if frosting is too thick, or an extra pinch of salt if frosting is too sweet.

Assembly:

1. Once cakes are fully cooled (or even better, slightly thawed), make sure the cakes are all about equal height. If some of your cakes have domed a bit (mine did), trim the tops so that you have nice, even layers.
2. Referencing the photos above as a guide, use a 6-inch round cookie cutter to cut a circle out of each cake. Using a 3-inch round cookie cutter, cut a circle out of each newly-formed 6-inch cake. Repeat with all of the cakes.
3. Start stacking the cakes by placing one 9-inch circle outline of a cake on the serving late, then insert an alternating color of the 6-inch cake, and then inserting a 3-inch round. Spread a thin layer frosting on top, and repeat with alternating colors on the next layer. Repeat until completed with all 4 layers, and then spread the remaining frosting on the top and sides. Decorate cake as desired!