1-First Step: Get the pans and oven ready Preheat the oven to 350°F (177°C). Grease three 9-inch round cake pans, line the bottoms with parchment paper rounds, and grease the parchment too. This small step helps the layers release cleanly, which matters when you are building a neat Birthday Sprinkles Cake. If you only have two pans, you can bake in batches, but three pans make the process smoother. For best results, bring the butter, eggs, egg whites, and buttermilk to room temperature before you begin.

2-Second Step: Mix the dry ingredients In a medium bowl, whisk together the cake flour, baking powder, baking soda, and salt. Set the bowl aside while you work on the wet ingredients. Cake flour matters here because it helps create a softer, more delicate crumb. If you are baking for gluten-free guests, use a tested 1:1 gluten-free flour blend in place of the cake flour, and check your sprinkles for gluten-free labeling as well.

3-Third Step: Cream the butter and sugar Beat the softened butter and granulated sugar on high speed for about 5 minutes, until the mixture looks fluffy and pale. Scrape down the bowl as needed. Then mix in the vegetable oil for about 1 minute. This blend of butter and oil helps the Birthday Sprinkles Cake stay rich without feeling too heavy. The butter adds flavor, while the oil supports moisture.

4-Fourth Step: Add the eggs and vanilla Add the whole eggs, egg whites, and vanilla extract. Beat on medium-high speed until everything is combined, scraping the bowl again so no streaks remain. Adding the egg whites with the whole eggs keeps the method simple and still gives the cake a light feel. If you want a more neutral white frosting later, use clear imitation vanilla in the buttercream. For a classic birthday flavor, pure vanilla works beautifully in both the cake and frosting.

5-Fifth Step: Add the dry ingredients and buttermilk Add the dry mixture and the buttermilk in three additions, alternating between the two and mixing just until each addition is incorporated. Do not overmix, since that can make the cake tough. Stop as soon as the batter looks smooth. If you need a homemade buttermilk substitute, mix 1 tablespoon vinegar or lemon juice with enough milk to make 1 1/2 cups total. Let it sit for 5 minutes before using.

6-Sixth Step: Fold in the sprinkles Gently fold in the rainbow sprinkles with a spatula. Use American-style jimmies for the best result, because they keep their color during baking. Avoid nonpareils, since they can bleed and muddy the batter. Divide the batter evenly among the prepared pans. Try to keep the layers equal so the finished cake stacks nicely and bakes evenly.

7-Seventh Step: Bake the cake layers Bake for 25 to 26 minutes. If you are using 8-inch pans, the bake time may be closer to 27 to 28 minutes. After 15 minutes, tent the pans loosely with foil to prevent over-browning on top. Test doneness with a toothpick in the center of each layer. For convection ovens, lower the temperature by 25°F and keep a close eye on the cakes. Every oven behaves a little differently, so begin checking early if yours runs hot.

8-Eighth Step: Cool the layers fully Cool the cakes in the pans on a wire rack for 20 minutes. Then remove them from the pans and let them cool completely on the racks. This is important because warm layers can make frosting slide or melt. If you plan ahead, you can bake the layers a day early, wrap them well, and frost the cake the next day. That works well for birthdays when your schedule is busy.

9-Ninth Step: Make the vanilla buttercream Beat the softened butter for about 2 minutes, until creamy. Add the confectioners’ sugar, heavy cream, vanilla extract, and salt. Beat on low for 30 seconds, then on high for 2 minutes. If the frosting feels too thick, add a little more cream. If it feels too soft, add a bit more sugar. You can tint the frosting with gel food coloring if you want a brighter birthday look. Keep the color light if you still want the confetti sprinkles to stand out.

10-Tenth Step: Assemble and frost the cake Level the tops of the cake layers if needed. Place one layer on a serving plate or cake stand and spread about 1 cup of frosting over the top. Repeat with the second and third layers. Then apply a thin crumb coat over the outside of the cake and chill it for 30 minutes. The crumb coat traps loose crumbs and helps the final frosting layer look smooth. After chilling, spread the remaining frosting on the top and sides.

11-Final Step: Decorate and serve Finish the Birthday Sprinkles Cake with extra sprinkles on top if you like. Serve it right away, or cover and keep it at room temperature for up to 4 hours. If you refrigerate it, bring it back to room temperature before serving so the texture softens and the flavor comes through. For the best taste, let a chilled cake sit out for about 30 minutes before slicing. That small wait makes a big difference in both texture and flavor.