Chocolate-Flecked Layer Cake with Milk Chocolate Frosting

INGREDIENTS

• 1 1/2 sticks unsalted butter, softened, plus more for pans
• 2 1/4 cups all-purpose flour, plus more for pans
• 1 1/2 teaspoons baking soda
• 2 1/2 teaspoons coarse salt
• 1 cup plus 2 tablespoons granulated sugar
• 1/3 cup packed light-brown sugar
• 1 large egg yolk plus 2 large eggs
• 1 tablespoon pure vanilla extract
• 1 1/2 cups sour cream
• 3/4 cup whole milk
• 8 ounces bittersweet chocolate (61 to 70 percent cacao), finely chopped
• Milk Chocolate Frosting