Preheat your oven to 350°F (175°C) and line a standard cupcake pan with 12 cupcake liners.

Whisk together cake flour, baking powder, baking soda, and salt in a bowl until combined.

Cream together unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes.

Beat in egg whites and vanilla extract until the mixture is pale and silky, about 1-2 minutes.

Gradually alternate adding the dry mixture and buttermilk until just combined.

Divide the batter among the cupcake liners, filling each about 2/3 to 3/4 full.

Bake for 16-18 minutes or until a toothpick comes out clean; cool for 10 minutes.

For the ganache, melt white chocolate, butter, and heavy cream in a double boiler, stir until smooth.

Whip the softened butter until pale and creamy, then gradually add melted white chocolate and powdered sugar.

Remove cupcake centers, fill with ganache, and pipe buttercream on top.