Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans or line them with parchment paper.

In a medium bowl, whisk together the cake flour, baking powder, and salt. Set aside.

In a large mixing bowl, beat the softened butter and sugar with an electric mixer on medium speed until light and fluffy, about 2–3 minutes.

Beat in the egg whites one at a time, scraping down the sides of the bowl as needed.

Reduce the mixer speed to low. Add the flour mixture in three parts, alternating with the milk in two parts (flour-milk-flour-milk-flour). Mix until just combined.

Fold in the melted white chocolate and the vanilla extract. Stir gently until the batter is smooth.

Divide the batter evenly between the prepared pans. Bake for 22–25 minutes or until a toothpick inserted into the center comes out clean.

Let the cakes cool in the pans for 10 minutes, then turn them out onto a cooling rack to cool completely.

In a large bowl, beat the softened butter until smooth.

Mix in the cooled, melted white chocolate.

Gradually add the powdered sugar, alternating with the milk or cream, until the desired consistency is reached.

Beat in the vanilla extract and salt. Adjust the thickness by adding more sugar or milk if needed.

Place one cake layer on a serving plate or cake stand. Spread an even layer of frosting on top.

Place the second layer over it and frost the top and sides of the cake. For a decorative touch, pipe extra frosting around the edges or add sprinkles.