Devil's Food Cake with Milk Chocolate Frosting

INGREDIENTS

• 3/4 cup sour cream
• 1 1/2 cups (3 sticks) unsalted butter, room temperature, plus more for pans
• 3/4 cup Dutch-process cocoa powder, sifted, plus more for pans
• 3/4 cup hot water
• 3 cups cake flour, (not self-rising), sifted
• 1 teaspoon baking soda
• 1/2 teaspoon salt
• 2 1/4 cups sugar
• 4 large eggs
• 1 tablespoon pure vanilla extract
• 8 ounces bittersweet chocolate
• 8 ounces milk chocolate
• 1 1/2 cups heavy cream
• 2 tablespoons corn syrup
• 3 sticks unsalted butter, soft but cool, cut into 1-inch pieces