1-First Step: Get the pans and oven ready Preheat your oven to 350°F, which is 175°C. Grease three 9-inch cake pans, line the bottoms with parchment paper, and then grease the parchment as well. This extra step helps the layers release cleanly, which is especially important for a soft birthday sprinkles cake. If you want to bake this as an 8-inch cake or a sheet cake, the batter adapts well, though baking time will change.

2-Second Step: Mix the dry ingredients In a medium bowl, whisk together the cake flour, baking powder, baking soda, and salt. Whisking helps spread the leavening evenly through the flour, so each layer rises nicely. Set this bowl aside while you prepare the butter mixture.

3-Third Step: Cream the butter and sugar In a large bowl, beat the softened butter and granulated sugar on high speed for about 5 minutes. The mixture should look creamy and a little fluffy. Add the vegetable oil and beat for 1 more minute. The oil gives the cake extra moisture, which helps the crumb stay soft after baking.

4-Fourth Step: Add the eggs and vanilla Add the 4 large eggs, the 2 egg whites, and the vanilla extract. Beat on medium high speed until everything is combined. The mixture may look a little curdled at first, but it should come together as you keep mixing. Using both whole eggs and egg whites keeps the cake rich while still light.

5-Fifth Step: Alternate the flour mixture and buttermilk Add the dry ingredients and buttermilk in three additions, starting and ending with the flour mixture. Mix just until each addition is incorporated. This gentle mixing matters because overmixing can make the cake dense. The batter should look smooth and thick, not stiff.

6-Sixth Step: Fold in the sprinkles Gently fold in the rainbow sprinkles with a spatula. Use jimmies or quins for the best result. Avoid nonpareils because they can bleed color into the batter and turn it muddy. Folding by hand helps keep the colors bright and the batter airy.

7-Seventh Step: Divide and bake Divide the batter evenly among the three prepared pans. Bake for 25 to 26 minutes. If the tops are browning too fast, tent the pans with foil after 15 minutes. The cakes are done when a toothpick inserted in the center comes out clean or with just a few moist crumbs. For a quick look at another celebratory layer cake idea, you might also enjoy my butter pecan layer cake.

8-Eighth Step: Cool the layers Let the cakes cool in the pans for 20 minutes. Then turn them out onto wire racks and let them cool completely. Do not rush this part. Warm cake can melt the frosting, and that makes assembly messy. Full cooling also gives you neater slices later.

9-Ninth Step: Make the vanilla buttercream Beat the softened butter until creamy. Add the confectioners’ sugar, heavy cream, vanilla, and salt. Beat until smooth and fluffy. If you want a more colorful finish, you can tint the frosting with gel food coloring. Vanilla extract gives the buttercream its familiar bakery style flavor, and you can read more about it in this helpful guide to vanilla extract benefits.

10-Tenth Step: Assemble the cake Level the cake layers if needed so they stack neatly. Place the first layer on a serving plate or cake stand and spread frosting on top. Add the second layer, frost again, then finish with the third layer. Apply a thin crumb coat over the whole cake and chill it for 30 minutes. This helps lock in crumbs and gives you a smoother final layer of frosting.

11-Final Step: Frost and finish Spread the remaining buttercream over the chilled cake. Smooth the sides and top, then garnish with additional sprinkles if you like. The finished birthday sprinkles cake is ready to slice, serve, and celebrate. It is especially lovely for birthdays, potlucks, and any day that needs a little color.