Preheat oven to 350 degrees. Prepare two 9-inch rounds.
Melt chocolate in double boiler. Beat egg whites until frothy. Gradually add in 1/2 cup sugar. Beat until stiff and glossy. Set aside.
Sift and measure flour. Sift again into large bowl with baking soda, salt and 1 cup sugar. Add oil and 1/2 of yogurt. Beat 1 minute. Add remaining yogurt, milk, egg yolks and melted chocolate. Beat 1 minute. Fold in beaten egg whites.
Pour into prepared pans. Bake 25 to 30 minutes or until toothpick inserted in center comes out dry. Cool on racks 15 minutes. Remove from pans and cool completely before frosting.
Frost with Superb Chocolate Frosting. Makes 12 to 16 servings.