Combine the egg whites, sugar, water and cream of tartar in the top of a double boiler. Mix thoroughly.
Place over boiling water and beat constantly until mixture holds a peak. This will take at least 7 minutes, but most likely closer to 10 minutes.
Remove from the heat.
Add vanilla and beat until thick enough to spread.
Makes 5-6 cups of frosting.
The calories shown are based on the recipe frosting a cake that serves 15, with 1 serving the frosting for one slice of cake (about 1/3 cup). Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**