1-First Step: Get the pans and oven ready Start by heating your oven to 350°F (177°C). Grease three 9-inch round cake pans, line the bottoms with parchment paper rounds, then grease the parchment too. This extra step may seem small, but it makes the cakes release cleanly and saves you from the heartbreak of a stuck layer. If you are using 8-inch pans instead, the batter still works, but the bake time will run a little longer. Set out the eggs, egg whites, butter, buttermilk, and heavy cream so they can come to room temperature. Room-temperature ingredients mix more smoothly and help the batter stay airy. If you need a gluten-free version, get your flour blend ready now so you can swap it in without slowing down later.

2-Second Step: Mix the dry ingredients In a large bowl, whisk together the cake flour, baking powder, baking soda, and salt. This step spreads the leaveners evenly through the flour, which helps the cake rise in a balanced way. Cake flour is a smart choice here because it gives the Birthday Sprinkles Cake a softer, finer crumb than regular all-purpose flour. Do not rush this part. A good whisking now means fewer pockets of baking soda later, and nobody wants a surprise bitter bite in a birthday cake.

3-Third Step: Cream the butter and sugar Using a stand mixer or hand mixer, beat the softened butter and granulated sugar on high speed for about 5 minutes until the mixture looks fluffy and pale. This step adds air to the batter and helps build a lighter texture. Next, mix in the vegetable oil for about 1 minute. The oil adds moisture, which keeps the cake soft even after chilling. Add the eggs, egg whites, and vanilla extract, then beat on medium-high speed until everything is combined. The whole eggs and extra egg whites work together to keep the crumb light while still giving the cake structure. If you prefer to read more about this flavor choice, vanilla is doing a lot of quiet heavy lifting here.

4-Fourth Step: Add the wet and dry ingredients Now add the flour mixture and the buttermilk in three alternating additions, mixing just until each addition disappears into the batter. The key here is not to overmix. Once the flour goes in, you want to keep the batter tender, not tough. After that, gently fold in the rainbow sprinkles. Use American-style jimmies for the best results because they hold their shape and color during baking. Avoid nonpareils since they tend to bleed and make the batter look muddy. A gentle hand is important here so the sprinkles stay bright and do not streak through the batter.

5-Fifth Step: Divide and bake Evenly divide the batter among the prepared pans. Smooth the tops with a spatula so the layers bake evenly. Bake for about 25 to 26 minutes for 9-inch pans, or 27 to 28 minutes if you are using 8-inch pans. After about 15 minutes, tent the pans loosely with foil if the tops are browning too fast. Check the cakes with a toothpick near the center. If it comes out clean or with just a few moist crumbs, the cakes are done. Every oven behaves a little differently, so keep an eye on the layers near the end of the bake time. If you use convection, lower the temperature by 25°F and start checking earlier.

6-Sixth Step: Cool the layers properly Let the cakes cool in the pans on a wire rack for about 20 minutes. Then carefully turn them out onto the racks and let them cool completely. Do not frost warm layers, or the buttercream will melt and slide right off. A fully cooled cake is much easier to stack, frost, and decorate. This is also a good time to make your buttercream if you have not already done so. The frosting can sit at room temperature for a little while, or you can refrigerate it and bring it back to a spreadable texture later.

7-Seventh Step: Make the vanilla buttercream Beat the softened butter on medium speed for 2 minutes until creamy. Add the confectioners’ sugar, heavy cream, vanilla extract, and salt. Mix on low for about 30 seconds so the sugar does not fly everywhere, then beat on high for 2 minutes until the frosting is smooth and fluffy. If the frosting seems too thick, add a little more cream. If it feels too soft, add a bit more sugar. You want a spreadable texture that holds its shape on the cake. For a brighter white frosting, use clear imitation vanilla instead of pure vanilla extract.

8-Eighth Step: Assemble the cake Use a serrated knife to level the cake tops if needed. Place one layer on your serving plate or cake stand and spread about 1 cup of frosting evenly over the top. Add the second layer and repeat. Place the final layer on top, then spread a thin crumb coat around the whole cake. Smooth the crumb coat with a spatula, then chill the cake for 30 minutes. This step locks in the crumbs so your final frosting layer stays neat. If you have ever watched cake crumbs sneak into the frosting like they own the place, you already know why this matters.

9-Ninth Step: Finish and decorate Spread the remaining frosting evenly over the sides and top of the cake. You can make the finish smooth and polished or go for a more relaxed homemade look. Add extra sprinkles on top if you want a stronger confetti effect. The frosting can also be tinted with gel food coloring if you want a themed birthday cake. For a cleaner slice later, let the cake chill briefly after frosting, then bring it back to room temperature before serving. This helps the buttercream soften and improves the flavor.

10-Tenth Step: Serve and store Serve the cake right away, or keep it at room temperature for up to 4 hours. If you need to store it longer, cover it and refrigerate it for up to 1 day before serving. Let it sit at room temperature for 30 minutes after chilling so the frosting softens again. Leftovers should be stored tightly covered in the refrigerator for up to 5 days.