Instructions: Preheat oven to 350 degrees. Butter and flour two 9- by 2-inch cake pans.

Melt chocolate in double boiler and set aside to cool. Drain beets and pure in a blender or food processor; you should have 1 cup. Sift together flour, cocoa, baking soda and salt.

In a bowl with an electric mixer, beat butter, sugars, eggs and vanilla until light and fluffy, about 5 minutes. Beat in chocolate. With mixer on low, alternately beat in flour mixture (in fourths) and buttermilk mixed with vinegar (in thirds), beginning and ending with flour. Beat until incorporated. Add pureed beets and mix on medium until blended.

Divide the batter between the pans. Bake for 30 to 35 minutes, or until cake tester inserted in center comes out clean and top of cake springs back when lightly pressed. Cool cakes in pans on wire racks for 10 minutes and invert onto racks to cool completely.

Make frosting: In a bowl set over a large saucepan of simmering water beat sugar, cream of tartar, salt, egg whites and water with mixer for 5 to 7 minutes, or until mixture holds stiff peaks. Remove bowl from pan and beat icing for 3 minutes more. Beat in vanilla.

To assemble cake: On serving plate spread one cake layer with a third of frosting and top with second layer. Spread remaining frosting over top and sides.

This recipe yields one 9-inch layer cake, 8 to 10 servings.

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