Place all of the ingredients except the vanilla in a double boiler over boiling water. Beat constantly with an electric mixer on medium-high speed until stiff peaks form, 4-7 minutes. Remove from heat.
Add the vanilla. Beat to spreading consistency.
Citrus Low-Fat 7-Minute Frosting: Replace the water with lemon, orange, or lime juice. Spoon in 1 tablespoon of zest.
Coffee Low-Fat 7-Minute Frosting: Replace the granulated sugar with brown sugar. Dissolve 2 tablespoons of instant coffee powder in 1/3 cup hot water and omit cold water.
Pepperming Low-Fat 7-Minute Frosting: Replace vanilla with 1/2 teaspoon peppermint extract. After frosting has cooled, fold in 1/3 cup crushed peppermint candies.
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