Marble Cake: Easily Make This Swirled Cake Design

Ingredients

• 4 ounces bittersweet chocolate, chopped
• 3 tablespoons plus 1-1/4 cups butter, softened, divided
• 2 cups sugar
• 5 large eggs, room temperature
• 3 teaspoons vanilla extract
• 2-1/4 cups all-purpose flour
• 2 teaspoons baking powder
• 1/2 teaspoon salt
• 1/2 cup sour cream
• 1/2 cup miniature semisweet chocolate chips, optional
• FROSTING:
• 3/4 cup butter, softened
• 6-3/4 cups confectioners' sugar
• 2 teaspoons vanilla extract
• 1/2 to 2/3 cup 2% milk
• 2 tablespoons miniature semisweet chocolate chips