Devil's Food Cake with Fluffy Frosting

INGREDIENTS

• 3 sticks unsalted butter, room temperature, plus more for pans
• 1 cup unsweetened cocoa powder, plus more for pans
• 1 cup boiling water
• 1 cup sour cream
• 3 cups all-purpose flour (spooned and leveled)
• 3/4 teaspoon fine salt
• 1 teaspoon baking powder
• 1 teaspoon baking soda
• 3 3/4 cups sugar, divided
• 4 large eggs, plus 5 large egg whites, room temperature, divided
• 5 teaspoons pure vanilla extract, divided
• 1 tablespoon corn syrup