a 10-inch greaseproof cake board; a piping bag fitted with your favorite frosting tip
For the cake: Preheat the oven to 350 degrees F. Line three 8-inch round cake pans or four 7-inch round cake pans with parchment rounds, and grease with nonstick cooking spray.
Mix together the flour, granulated sugar, baking powder and salt in a stand mixer with the paddle attachment until fully combined. Mix chunks of butter slowly into the dry mixture on low speed until no large chunks of butter remain and the mixture becomes crumbly. Pour in the egg whites and mix on low until just incorporated. Mix in the buttermilk, then add the vanilla and oil and mix until fully incorporated. Scrape down the sides of the bowl as needed.
Add the gel food coloring, if using, then fold in by hand with a rubber spatula, scraping the sides and bottom of the bowl partway through, until the batter is evenly colored. Divide the batter evenly among the prepared cake pans.
Bake until a toothpick inserted in the center of a cake layer comes out clean, 34 to 35 minutes. Allow the pans to cool for 10 minutes, then run an offset spatula around perimeters of the pans to separate the layers from the pans.
Place the cake layers in their pans in the freezer until fully cooled, 45 minutes to 1 hour. Carefully flip the pans and remove the layers from the pans. Use a serrated knife to level the tops of the layers.
For the vanilla buttercream: Meanwhile, beat the butter in a stand mixer fitted with a paddle attachment on medium speed until smooth, about 30 seconds. Turn the mixer to low speed and slowly add the powdered sugar 1 cup at a time, alternating with small splashes of cream, until fully mixed. Add the vanilla and salt and beat until the ingredients are fully incorporated and the desired consistency is reached. (If the frosting is too thick, add additional cream 1 tablespoon at a time. If the frosting is too thin, add more powdered sugar 1/4 cup at a time.) Add the gel food coloring, if using, and beat on low until it reaches the desired color.
Stack a cake layer on a greaseproof cake board using a dab of frosting to help it stick.
Frost the top with an even layer of buttercream. Continue to stack the layers, adding a thin layer of buttercream on top of each.
Apply a thin coat of frosting around the sides and on top of the cake, to fully cover the cake layers. Smooth using a bench scraper, then chill the cake until the frosting is firm to the touch, 20 minutes in the fridge or 5 minutes in the freezer.
Place the remaining frosting in a piping bag fit with your favorite frosting tip, and pipe large swirls around the perimeter of the top of the cake.