For the cake:
1 cup all-purpose flour
2 teaspoons unsweetened cocoa powder
3/4 teaspoon baking powder
1/2 teaspoon baking soda
1/2 teaspoon salt
4 large eggs
3/4 cup sugar
2 tablespoons melted butter, cooled but still liquid
1 teaspoon vanilla extract
2 tablespoons buttermilk
1 teaspoon white vinegar
Red food coloring

For the peppermint buttercream:
1/2 cup (4 ounces) egg whites, from about 4 large eggs
1 1/3 cup granulated sugar
2 cups unsalted butter, softened
3 1/2 ounces white chocolate, melted
3/4 teaspoon peppermint extract, or to taste
1 teaspoon vanilla extract