1-First Step: Prep and dry ingredients Preheat oven to 350°F (177°C). Grease three 9-inch round cake pans and line with parchment paper rounds, then lightly grease the parchment. This helps the layers release cleanly. Whisk together 3 and 3/4 cups (443g) cake flour, 1 teaspoon baking powder, 1 teaspoon baking soda, and 1 teaspoon salt in a large bowl; set aside.

2-Second Step: Cream butter and sugar In the bowl of a stand mixer fitted with the paddle attachment, beat 1 and 1/4 cups (282g) softened unsalted butter and 2 cups (400g) granulated sugar on high for 5 minutes until fluffy and pale. Scrape down the sides of the bowl as needed. Add 1/3 cup (80ml) vegetable oil and beat for 1 minute to combine; this improves moisture while keeping structure.

3-Third Step: Add eggs and flavor Add 4 large eggs and 2 large egg whites, both at room temperature, plus 3 teaspoons (15ml) pure vanilla extract. Beat on medium-high until fully combined and the mixture looks smooth. Scrape the bowl again to keep everything even.

4-Fourth Step: Combine dry ingredients and buttermilk Add the dry ingredients and 1 and 1/2 cups (360ml) buttermilk alternately in three additions, beginning and ending with the dry mixture. Mix only until incorporated – overmixing can tighten the crumb. The batter will be thick and velvety. Gently fold in 3/4 cup (135g) rainbow sprinkles with a spatula so they stay intact and do not bleed color.

5-Fifth Step: Fill pans and bake Divide batter evenly between the three prepared pans. Bake at 350°F (177°C) for 25-26 minutes, or 27-28 minutes if using 8-inch pans, until a toothpick inserted into the center comes out clean. After the first 15 minutes of baking, tent each pan loosely with foil to prevent over-browning on top while the center finishes baking.

6-Sixth Step: Cool layers Cool cakes in the pans on a wire rack for 20 minutes, then run a knife around the edges and remove layers to cool completely on wire racks before frosting. Cooling completely helps prevent the buttercream from melting when applied.

7-Seventh Step: Make the vanilla buttercream For the frosting, beat 1 and 1/2 cups (340g) softened unsalted butter on medium speed for 2 minutes until creamy. Add 5 and 1/2 cups (650g) confectioners’ sugar, 1/3 cup (80ml) heavy cream, 3 teaspoons (15ml) vanilla extract (use clear imitation vanilla for a very white frosting), and 1/8 teaspoon salt. Beat on low for 30 seconds to combine, then increase to high and beat for 2 minutes until light and fluffy. Adjust thickness with more sugar to thicken or more cream to loosen.

8-Eighth Step: Level, stack, and crumb coat Level cake tops with a serrated knife or cake leveler for even stacking. Place the first layer on a cake stand or plate and spread about 1 cup of frosting evenly across the top. Repeat with the second layer. Apply a thin crumb coat of frosting around the whole cake to capture crumbs, then chill the cake for 30 minutes to 3 hours to set the crumb coat.

9-Final Step: Finish and decorate Spread the remaining frosting evenly on the chilled cake, smoothing with an offset spatula. Decorate with additional sprinkles as desired and serve immediately, or follow the storage guidance below for keeping the cake fresh. Leftovers store tightly covered in the refrigerator up to 5 days. Bring refrigerated cake to room temperature 30 minutes before serving for best texture.