1. Heat oven to 350 degrees 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, 1-1/2 cups 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::
2. Combine egg whites, 1-1/2 cups 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.
3. Spread cupcakes with warm frosting. Sprinkle lightly with cocoa, if desired.