Easily adapt a classic two-layer vanilla cake to make a delicate four-layer version simply by slicing each cake baked in a pan and slicing it in half horizontally, thus producing 4 layers that are ready to be filled and surrounded with vanilla buttercream frosting.
Allow the butter and the eggs (if chilled) to stand at room temperature for at least 30 minutes.
Get the ingredients ready for use by weighing/measuring each ingredient into separate bowls.
Cut two pieces of parchment paper into circles that just fit into the bottom of the cake pans. Cut another four rectangular pieces of parchment about 10 inches (25 cm) long and 2 inches (5 cm) wide.
Make the Cake
Preheat oven to 350°F (175°C).
Grease the bottom of the pans with about 1/4 teaspoon of butter each. Place a parchment paper circles into the bottom of each pan and grease the tops of the parchment paper and sides of each pan with about 1/2 teaspoon of butter. Add 1 tablespoon of flour into one pan, flip the other pan over to cover the first pan and lightly shake the mutually covered pans to coat their insides with the flour.
In the medium mixing bowl, add the flour, baking soda, baking powder and salt and stir with the wire whisk until thoroughly combined.
In the large mixing bowl, add the butter and beat with the stand mixer of hand mixer on medium to high speed for 30 seconds. Gradually add the sugar to the butter, a quarter cup at a time, each time beating on medium speed until well combined before adding the next quarter cup, scraping the sides of the bowl down occasionally with the scraper spatula. Continue to beat for an additional 2 minutes.
Add the vanilla and beat until combined. Add the egg whites one at a time, beating and scraping the sides of the bowl after each addition. Alternately add a third of the flour mixture and a third of buttermilk/sour milk, beating on low speed after each addition until just combined.
Pour the batter evenly into the greased and floured pans.
Bake for 20 to 25 minutes or until a wooden toothpick inserted near the center of each pan comes out clean, turning and swapping the positions of the pans halfway through the baking.
While the cake is baking, prepare the frosting (see below).
Cool the baked pans on a wire rack for 10 minutes. Remove the cake layers from the pans, remove the parchment paper and cool them completely on the wire rack.
Frost the Cake
If the cake layer has a domed surface, use a bread knife or a long, thin knife to cut away the dome top. Then, gently half the cake layer horizontally into two so that you end up with a total of 4 cake layers from the 2 pans, each with flat surfaces ready for frosting to be applied. Reserve the 2 cake layers that were at the bottom of the pans to become the bottom and top of the four-layer cake.
On a cake board or cake stand (preferably both, with a cake board placed in the center of the cake stand), place 2 tablespoons of frosting in the center and spread it into a small circle of about a couple of inches in diameter.
Place one of the reserved cake layers on the cake board/cake stand and gently use the pastry brush to brush away any loose crumbs. Gently tuck the four rectangular pieces of parchments just under the edge of the cake layer, overlapping one another to help keep the cake board/cake stand clean during frosting.
Starting from the center and moving to the edge, evenly spread about 1/2 cup of the frosting on the top of the cake layer.
Gently place one of the non-reserved cake layers onto the first cake layer and gently use the pastry brush to brush away any loose crumbs. Starting from the center and moving to the edge, evenly spread about 1/2 cup of the frosting on the top of the cake layer.
Gently place another one of the non-reserved cake layers onto the second cake layer and gently use the pastry brush to brush away any loose crumbs. Starting from the center and moving to the edge, evenly spread about 1/2 cup of the frosting on the top of the cake layer.
Gently place the final reserved cake layer top side down onto the third cake layer and gently use the pastry brush to brush away any loose crumbs. Starting from the center and moving to the edge, evenly spread about 1/2 cup of the frosting on the top of the cake layer.
Working in quarter sections, spread a thin layer of frosting onto the sides of the cake. Evenly spread about 1 to 1 1/2 cups of frosting on the top of the cake, starting from the center. Add more frosting to the sides, again working in quarter sections, blending the edges with the frosting on the top.
Slice and serve the cake immediately or chill in the refrigerator to be sliced and served later.
In the large mixing bowl (washed and dried after being used to make the cake batter, of course), beat the 3/4 cup butter with the stand mixer or electric hand mixer on medium speed until smooth.
Gradually add about 2 cups of the powdered sugar and beat until thoroughly combined. Add the milk and vanilla and beat well, scraping the sides of the bowl occasionally. Gradually beat in the remainder of the powdered sugar.
Test the consistency of the frosting (a tablespoon of frosting taken out of the mixing bowl should be thick enough yet still easy to spread without breaking). If it is too thick, beat in a teaspoon of milk at a time until the frosting has the right consistency.
Sour Milk Preparation
In a glass measuring cup, add 4 teaspoons of lemon juice or white vinegar. Add fresh whole milk until you can a total of about 1 1/3 cup (10 1/2 oz or 302 g) in the cup.
Let the mixture stand for at least 5 minutes before using.
Equipment Required (in order of use)
Up to 12 extra bowls to hold the ingredients
Kitchen weighing scale (preferably digital)
Measuring spoons & cups
Baking parchment paper
2 9-inch round cake pans
Medium mixing bowl
Large mixing bowl
Stand mixer or hand mixer with wire whisk attachment
Silicone scraper spatula
Bread knife or long thin knife
Cake board and/or cake stand (preferably one that has a turn table)