Preheat oven 350°F. Lightly butter and flour four 6-inch round cake pans. Set aside.
In a medium bowl, sift together the flour, sugar, baking powder, and salt. Set aside.
4 cups cake flour, 2 cups granulated sugar, 1 tablespoon baking powder, 1 teaspoon kosher salt
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium-high until smooth, about 2 minutes. Add in the vanilla and egg whites.
1 cup unsalted butter, 1 teaspoon pure vanilla extract, 8 large egg whites
Turn the speed up to high for about 30-60 seconds, or until the egg whites begin to foam.
Add the flour mixture and milk, continuously mixing on low speed, scraping down the sides as needed.
1½ cups milk
Once the batter is smooth and creamy, divide it evenly between the prepared cake pans (about 436 grams per pan).
Bake for 25-28 minutes, using a toothpick to check for doneness. Let the cakes cool in their pans for about five minutes and then remove from the pans to let them cool on a rack.
Before assembling, cut the tops off of each layer to have even and flat layers.
Wrap and freeze the cakes for about 2 hours before frosting.
To assemble the cake, add about ½ cup of frosting around the top of the first layer of cake, evenly spreading over the top of the cake. Top with the next cake layer, and gently press it into the bottom layer. Repeat with the next cake until all 4 layers are stacked with a layer of frosting in between each layer.
2 cups vanilla buttercream frosting
Using a flat spatula, gently spread a thin layer of frosting over the entire cake, using a cake scraper to remove excess.
Top the cake with fresh edible flowers, a cake topper, or fresh fruit dipped in sugar.