Dark-Chocolate Cake with Ganache Frosting

INGREDIENTS

• 1 cup (2 sticks) unsalted butter, room temperature, plus more for pans
• 1/2 cup unsweetened Dutch-process cocoa (spooned and leveled), plus more for pans
• 2 cups all-purpose flour (spooned and leveled)
• 1/2 teaspoon baking powder
• 1/2 teaspoon baking soda
• 1/2 teaspoon salt
• 1 1/2 cups packed light-brown sugar
• 2 large eggs plus 2 large egg yolks, room temperature
• 6 ounces bittersweet chocolate, melted
• 1 teaspoon pure vanilla extract
• 1 cup low-fat buttermilk
• Dark-Chocolate Ganache