Good-Cooking - Brownstone Front Cake

Ingredients

• 1 cup buttermilk
• 1 teaspoon baking soda
• 1 cup (2 sticks) margarine
• 2 cups granulated sugar
• 3 egg yolks
• 2 squares unsweetened chocolate, melted
• 3 cups cake flour
• 1/2 teaspoon salt
• 1 teaspoon vanilla extract
• 3 egg whites, stiffly beaten