1-First, preheat your oven to 350°F (175°C) and get three 9-inch cake pans ready by greasing them, lining with parchment paper, and greasing the paper too. In a large bowl, whisk together the cake flour, baking powder, baking soda, and salt to blend the dry ingredients well.

2-Next, in a mixer, beat the softened butter and granulated sugar on high speed for about 5 minutes until it’s creamy and light. Add the vegetable oil and beat for another minute to mix it in smoothly. Then, add the eggs, egg whites, and vanilla extract, mixing on medium-high speed until everything combines nicely.

3-After that, alternately add the dry ingredients and buttermilk in three parts, mixing just until incorporated to keep the batter light and airy. Gently fold in the rainbow sprinkles so they spread evenly without messing up the color. Divide the batter evenly among the pans and bake for 25 to 26 minutes, or until a toothpick comes out clean.

4-Once baked, let the cakes cool in the pans for 20 minutes before moving them to wire racks to cool completely. For the frosting, beat the softened butter until creamy, then gradually add confectioners sugar, heavy cream, vanilla extract, and salt, beating until smooth and fluffy. To finish, level the cakes, spread frosting between layers, apply a thin crumb coat, chill for 30 minutes, and add the final frosting layer with extra sprinkles if you like.