For the Vanilla Cake

• Preheat the oven to 350 and line 3 8″ round cake pans with parchment paper.
• In the bowl of a stand mixer with the paddle attachment, cream the butter, sugar, and oil for 7 minutes. Scrape the bottom and sides of the bowl as needed.
• While the mixer is going, whisk the flour, cornstarch, baking powder, and salt, in a separate medium sized mixing bowl. Set aside.
• Stir the vanilla extract into the buttermilk. Set aside.
• In a small bowl, whisk the eggs and egg whites, then with the mixer on low, slowly incorporate the eggs into the batter. Once all the eggs are added, mix for 1-2 minutes, until smooth and glossy. Scrape the bottom of the bowl to make sure no egg has settled in the bottom of the bowl.
• Alternate adding the dry ingredients and buttermilk in 5 total additions, starting and ending with the dry ingredients.
• Once incorporated, do a final mix with a rubber spatula, then evenly divide the batter into the prepared pans.
• Bake at 350 for 25-27 minutes, or until a toothpick comes out mostly clean with a few moist crumbs attached. 
• Let cool for 5 minutes, then flip cakes out onto cooling rack, let cool completely, then wrap in plastic and place in the fridge to chill.

For the Vanilla American Buttercream

• Let the butter sit on the counter for 30-45 minutes before starting, but it shouldn’t be completely room temperature or too soft.
• Cream the butter in a bowl of a stand mixer with the paddle attachment for 10 minutes, scraping down the bowl every 2-3 minutes.
• While the butter mixes, sift the powdered sugar.
• Add the vanilla and mix to combine.
• Add the powdered sugar 1/2 at a time, mixing on medium low speed for 3 minutes after each addition.
• Add the heavy cream 2 TBSP at a time until the buttercream reaches your desired consistency.
• Do a final quick mix with a rubber spatula to release any remaining air bubbles.

Assembly

• Spread 2 tsp of buttercream onto a cake board or cake stand on top of a turn table.
• Place the first cake layer in the center of the board. This cake bakes up perfectly flat and does not dome when the instructions are followed closely. If your cake domes, make sure to level it off before stacking the cake.
• Spread buttercream in a thin even layer, then stack the second layer. 
• Repeat until all three layers are stacked.
• Apply the crumb coat, and smooth.
• Chill cake for 30 minutes to allow the crumb coat to set.
• Apply the final layer of buttercream and smooth. Find tips and detailed instructions on this up in the post.
• Apply rainbow sprinkles in random pattern on the sides of the cake.
• Pipe extra buttercream around the top edge of the cake using a piping bag and Wilton tip 8B.
• Add extra rainbow sprinkles to top.
• Slice and serve.