Cake

• Unsalted butter, softened, for pan
• ¾ cup Dutch-process cocoa powder, plus more for pan
• 1 ⅔ cups cake flour (not self-rising)
• 1 ⅓ cups sugar
• ¾ teaspoon baking powder
• ¾ teaspoon baking soda
• 1 ¼ teaspoons kosher salt (we use Diamond Crystal)
• 2 large eggs, room temperature
• ¾ cup buttermilk, room temperature
• ¾ cup hot tap water
• 2 teaspoons pure vanilla paste or extract
• 6 tablespoons vegetable oil

Filling

• ⅓ cup sugar
• 2 tablespoons cornstarch
• 1 tablespoon Dutch-process cocoa powder
• ¼ teaspoon kosher salt
• 1 ½ cups whole milk
• 2 large egg yolks (whites reserved for frosting)
• 1 teaspoon pure vanilla paste or extract
• 4 ounces milk chocolate, coarsely chopped (⅔ cup)
• 2 tablespoons unsalted butter

Frosting

• 3 large egg whites
• ¾ cup sugar
• Pinch of kosher salt
• 1 ½ sticks unsalted butter, cut into tablespoons, softened
• 3 ounces white chocolate, chopped (½ cup), melted, and cooled