1- Creating this Yellow Butter Birthday Cake starts with preheating your oven and preparing your tools it’s that simple and rewarding. First, preheat oven to 350°F and butter and flour three 6-inch round pans to prevent sticking and ensure even baking. Then, in a bowl, whisk together the flour (270g), corn starch (35g), baking powder, and salt; set aside for later use.

2- Next, cream the unsalted butter (113g, room temperature) and granulated sugar until the mixture is light and fluffy, which helps create a tender crumb. Add the vegetable oil (60mL) and vanilla extract (15mL), mixing well while scraping the bowl to incorporate everything evenly. Follow by adding the eggs and yolks one at a time, mixing after each addition and scraping down the sides to keep the batter smooth.

3- Alternate adding the flour mixture and buttermilk (240mL) in batches, mixing on low speed until just combined to avoid over-mixing, which can make the cake tough. Divide the batter evenly between the prepared pans and bake for about 30 minutes, or until the centers are set and springy when touched.

4- Baking and Finishing Steps Once baked, cool the cakes for a few minutes in the pans before inverting them onto wire racks to cool completely this step preserves the moist texture. For the chocolate buttercream, melt ¼ cup unsalted butter and mix in cocoa powder; set aside. Also, melt the semisweet chocolate with ¼ cup milk and set aside.

5- Beat 1 cup unsalted butter (room temperature) until fluffy, then add the melted butter and cocoa mixture, mixing well and scraping down. Gradually mix in the powdered sugar (450g) on low speed, followed by the chocolate mixture and the remaining milk, drizzling it in until smooth. Finally, assemble the cake by piping or spreading the buttercream on the first layer, stacking the remaining layers, and covering the cake completely with smooth or rustic swirls.