Favorite Chocolate Sheet Cake

Ingredients

• 1 cup butter, softened
• 2 cups sugar
• 4 eggs
• 2 teaspoons vanilla extract
• 2-1/4 cups cake flour
• 1 teaspoon baking soda
• 1 teaspoon salt
• 1 cup buttermilk
• 3 squares (1 ounce each) bittersweet chocolate, melted
• 1/4 cup baking cocoa
• 1/3 cup milk
• 1/2 cup butter, cubed
• 1 teaspoon vanilla extract
• 3-1/2 cups confectioners' sugar