1-First Step: Gather and prep Preheat oven to 350°F. Grease three 9-inch pans or three 8-inch pans and line each with parchment paper rounds. Leave ingredients at room temperature: eggs, extra egg whites, and buttermilk. Measure and sift the dry ingredients, and set the sprinkles aside in a small bowl so they are ready to fold in at the end.

2-Second Step: Mix dry ingredients In a medium bowl, whisk together 3 3/4 cups cake flour, 1 teaspoon baking powder, 1 teaspoon baking soda, and 1 teaspoon salt until evenly combined. Sifting is optional but helps with even distribution and a lighter batter.

3-Third Step: Cream butter and sugar In the bowl of a stand mixer fitted with the paddle attachment (or using a large bowl and a hand mixer), beat 1 1/4 cups softened unsalted butter and 2 cups granulated sugar on high speed for 5 minutes until very light and fluffy. This aeration step gives the cake lift and a fine crumb. Add 1/3 cup vegetable oil and beat for 1 minute to incorporate added moisture.

4-Fourth Step: Add eggs and vanilla Add 4 large room-temperature eggs and 2 large room-temperature egg whites to the butter-sugar mixture, plus 3 teaspoons pure vanilla extract. Mix on medium speed until combined. Scrape the bowl periodically to keep the batter uniform.

5-Fifth Step: Combine dry and wet components With the mixer on low, add the dry ingredients and 1 1/2 cups buttermilk in three alternating additions, beginning and ending with the dry ingredients. Mix just until incorporated after each addition; do not overmix. This alternating method helps keep the batter tender and prevents gluten overdevelopment.

6-Sixth Step: Fold in sprinkles and divide batter Gently fold in 3/4 cup rainbow sprinkles with a spatula. Stir slowly and briefly so the sprinkles do not bleed color into the batter. Divide the batter evenly among the three prepared pans, smoothing the tops.

7-Seventh Step: Bake and monitor Bake at 350°F for 25-26 minutes for 9-inch pans (8-inch pans may take 27-28 minutes). After 15 minutes, tent the pans with foil if the tops are browning too quickly. Test doneness with a toothpick inserted into the center; it should come out clean or with a few moist crumbs. Remove from the oven and cool in the pans on wire racks for 20 minutes, then run a thin knife around the edges and invert to remove cakes. Cool completely on racks before frosting.

8-Eighth Step: Make the vanilla buttercream In a clean bowl, beat 1 1/2 cups softened unsalted butter until creamy. Reduce speed and add 5 1/2 cups confectioners’ sugar a cup at a time, then pour in 1/3 cup heavy cream, 3 teaspoons vanilla extract (or clear imitation vanilla for white frosting), and 1/8 teaspoon salt. Start mixing on low to avoid a sugar cloud, then increase to high and beat until light and fluffy, about 3-4 minutes. Adjust consistency with more cream to thin or more sugar to thicken.

9-Ninth Step: Assemble and crumb-coat Trim the domes from cake layers to level them if needed. Place the first layer on a cake board or serving plate and spread an even layer of buttercream. Add the second layer and repeat, then top with the third layer. Apply a thin crumb coat of frosting around the sides and top to trap crumbs. Chill the cake for 30 minutes in the fridge to set the crumb coat.

10-Tenth Step: Final frosting and garnish Remove the chilled cake and apply the final layer of buttercream, smoothing with an offset spatula or cake scraper. Decorate as desired with optional additional sprinkles around the top edge or pressed into the sides. Pipe borders if you like. Serve immediately or follow storage notes below.