Heat oven to 350 degree F. Line twenty-four 2-1/2-inch muffin-pan cups with paper liners. Combine milk and vinegar in a 1-cup measure. Let stand 5 minutes. Combine flour, sugar, cocoa, baking powder, baking soda, and salt in a large mixer bowl. Beat in butter at low speed until mixture is crumbly. Add milk mixture, eggs, and vanilla; beat at medium speed 3 minutes. Spoon about 1/4 cup batter into each paper liner. Bake 18 to 20 minutes, until top springs back when lightly touched. Cool in pan on a wire rack 5 minutes. Remove from pan and cool completely.

Make Fluffy White Frosting: Combine egg whites, sugar, cream of tartar, and water in top of a double boiler. Beat on high speed 1 minute with hand held mixer. Place over boiling water (water should not touch bottom of pan); beat on high speed 7 minutes. Remove pan from boiling water; add vanilla and beat on high 2 minutes more, until stiff enough to spread. Makes 2-3/4 cups.

Spread cupcakes with warm frosting. Sprinkle lightly with cocoa, if desired. Makes 24 cupcakes.

Prepare batter as above. Grease 13x9-inch baking pan. Line bottom with waxed paper; grease and flour paper and sides. Pour batter into prepared pan. Bake 30 to 35 minutes, until top springs back when lightly touched. Cool in pan on rack 10 minutes. Invert cake; remove paper and cool completely. Frost cake with warm frosting. Decorate with cocoa, if desired. Makes 24 slices.