Notes: If you are concerned about egg safety, use a thermometer to check frosting temperature as it cooks; it should be between 140° and 150° for 3 minutes. A double boiler is most effective for heating the mixture evenly.
1. Use a double boiler, or snugly nest a 2- to 3-quart metal bowl in a 2- to 3-quart pan. Add 1 inch water to base of double boiler or pan. Bring to boiling over high heat.
2. Meanwhile, in double boiler top or bowl, combine egg white, sugar, 2 tablespoons water, and cream of tartar. Beat with a portable mixer until blended. Nest over boiling water; immediately reduce heat to maintain a gentle simmer. With mixer on medium speed, beat icing until it holds peaks that curve slightly, 5 to 7 minutes (overcooked, the icing gets grainy).
3. At once, remove icing from hot water. Mix in vanilla. Use, or if making up to 1 hour ahead, set pan or bowl in cool water to stop cooking, and cover airtight.
Nutritional analysis per 1/4 cup.