Spray the four pans, line the bottoms with parchment rounds, and spray the parchment. The pans are ready when the parchment lies flat.

Whisk the flour, sugar, baking powder, and 1 1/2 teaspoons salt in the mixer bowl on low for 30 seconds. The mixture looks even and pale.

Add the cake butter in 1 tablespoon pieces and mix on low for 2 minutes, until it looks like damp sand.

Whisk the 9 egg whites, milk, sour cream, 1 tablespoon vanilla, and almond extract in a large bowl for 30 seconds. The liquid looks smooth and pale.

Add half the liquid and mix on low for 20 seconds, then add the rest and mix on medium for 1 minute. The batter looks thick and glossy.

Weigh about 24 ounces of batter into each 8-inch pan and about 16 ounces into each 6-inch pan, then tap each pan twice. The batter sits about halfway up the pan.

Bake at 350°F with the pans staggered on both racks, and swap racks at 20 minutes. Pull the 6-inch layers at 25 to 30 minutes and the 8-inch layers at 30 to 35 minutes, when the tops spring back.

Cool the layers in the pans for 15 minutes, then turn them onto racks and cool 45 more minutes. The centers feel cool.

Wrap each layer in plastic and chill for 1 hour. The layers feel firm.

Trim the domes with a serrated knife. Every layer sits flat.

Whisk the 8 egg whites and 2 1/4 cups sugar in the clean mixer bowl over barely simmering water until the mixture reaches 160°F, about 5 to 8 minutes. No grit remains when you rub it between your fingers.

Whip the meringue on medium-high for 8 to 10 minutes. It forms stiff, glossy peaks, and the bowl feels cool.

Add the buttercream butter 1 tablespoon at a time on medium-low, then whip 5 to 8 minutes with 1/2 teaspoon salt and 1 tablespoon vanilla. The buttercream turns silky. If it looks curdled, warm the bowl with a hot towel for 20 seconds, or chill a soupy bowl for 10 minutes, then whip again.

Stack the 8-inch layers on an 8-inch cardboard round with 1 cup buttercream between them, and the 6-inch layers on a 6-inch round with 3/4 cup. The tiers sit level.

Spread a thin crumb coat over each tier, then chill for 30 minutes. The buttercream feels firm and matte.

Add a final coat and smooth the sides with the bench scraper while turning the stand. Chill for 20 minutes until the surface is firm.

Push four dowels down into the 8-inch tier inside the 6-inch circle, trim them level, and set the small tier on top. Push a sharpened dowel through the center of both tiers. The top tier sits level.

Pipe a shell border on each tier, add food-safe flowers, and chill for 30 minutes. Rest the cake at room temperature for 1 hour before serving, until the buttercream turns creamy.