1-First, preheat the oven to 350°F (177°C) and grease and line three 9-inch cake pans to prevent sticking. In a bowl, whisk together the cake flour, baking powder, baking soda, and salt until well combined. In a separate large bowl, beat the softened butter and sugar until creamy and light.

2-Next, add the vegetable oil, eggs, egg whites, and vanilla extract to the butter mixture, blending everything thoroughly. Alternate adding the dry ingredients and buttermilk to the wet mixture, mixing gently until fully incorporated to keep the batter airy. Gently fold in the rainbow sprinkles to maintain their vibrant colors, then divide the batter evenly between the prepared pans.

3-Bake for 25 to 26 minutes, or until a toothpick inserted in the center comes out clean. Allow the cakes to cool completely on a wire rack before moving on. For the vanilla buttercream, beat the softened butter until smooth, then gradually add the confectioners’ sugar, heavy cream, vanilla extract, and salt, beating until fluffy.

4-Assembling Your Cake: To assemble, layer the cakes with frosting between each layer and apply a crumb coat to seal in crumbs, then chill the cake for a bit. Add the final layer of frosting and garnish with extra sprinkles if desired. The total preparation time is about 45 minutes, with baking at 25 minutes, and overall around 4 hours including cooling and chilling.