Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, lining bottoms with parchment paper.

Melt 8 ounces of white chocolate in a microwave-safe bowl or double boiler, stirring until smooth. Cool slightly.

Whisk together 4 egg whites, 1/4 cup of milk, and 1 teaspoon of vanilla extract until frothy.

In a large bowl, combine 2 cups of sifted cake flour, 1 1/2 cups of sugar, 2 teaspoons of baking powder, and 1/2 teaspoon of salt. Whisk well.

Cream 1/2 cup of softened butter with dry ingredients for about 2-3 minutes, add 1 cup of milk, then fold in egg white mixture and melted white chocolate.

Divide batter between prepared pans, smooth the tops, and bake for 25-35 minutes until a toothpick comes out clean.

Cool cakes in pans for 10-15 minutes, then invert onto wire racks to cool completely.

Layer the first cake on a plate, spread white chocolate mousse, add the second layer, and cover with Italian meringue buttercream. Chill for 30 minutes.

For the ganache, heat 1 cup of heavy cream until simmering, pour over 8 ounces of chopped white chocolate, stir until smooth.

Drizzle ganache over the chilled cake, adding decorative swirls with remaining buttercream.