Prepare Your Pans - Preheat the oven to 350°F (177°C). Spray three 6-inch, two 8-inch, or two 9-inch pans with baking spray. Line the bottoms with parchment paper.

Mix the Dry Ingredients - In a medium bowl, whisk together 2¼ cups sifted cake flour, 2 teaspoons baking powder, ½ teaspoon baking soda, and ½ teaspoon salt. Set aside.

Cream the Butter and Sugar - Beat ½ cup room-temperature unsalted butter on high for 2 minutes until smooth. Add 1½ cups granulated sugar and beat on medium-high for 2 minutes until light and fluffy. Scrape the bowl as needed.

Add the Egg Whites, Oil, and Vanilla - Add 4 large egg whites one at a time, mixing after each addition. Add ¼ cup vegetable oil and 2 teaspoons vanilla extract, then beat on high for 1 minute.

Combine the Dry Ingredients and Buttermilk - Add the dry ingredients and mix on low. Slowly pour in 1 cup buttermilk and mix for about 30 seconds, just until the batter is smooth. Do not overmix.

Bake the Layers - Divide the batter evenly between the prepared pans and smooth the tops. Bake at 350°F (177°C) for 25–28 minutes for 6-inch layers or 28–32 minutes for 8- or 9-inch layers. A toothpick should come out clean or with a few moist crumbs.

Cool Completely - Remove the cakes from the pans and place them on a wire rack. Let them cool completely before frosting.

Level and Stack the Cake Layers - Trim any domed tops with a serrated knife. Place one layer on a cake plate, spread with frosting, then add the next layer. Repeat with the remaining layers.

Frost and Serve - Cover the top and sides with the remaining frosting and smooth with a spatula. Add berries, sprinkles, or simple vanilla frosting decorations before serving.