In a small saucepan over medium heat, bring sugar and water to a boil. Continue to boil until sugar syrup reaches 240-244 degrees F, soft-ball stage on a candy thermometer. As it cooks, start step 2, the meringue.
While sugar syrup is cooking, separate eggs, placing whites in the bowl of an electric mixer fitted with the whisk attachment. Beat on low to medium speed until foamy. Add cream of tartar, and whip on medium to high speed until medium to stiff peaks form, but not dry.
With mixer running, add cooked syrup to whites in a slow but steady stream, beating on high speed. Always be careful when working with hot sugar. Beat on high speed until steam from hot syrup stops, about 5 minutes. Add butter a tablespoon at a time, beating until smooth, approximately 4-5 minutes. Beat in vanilla, scrape down sides gently and beat again for 1 minute.
Keeps 3 days at room temperature and up to 2 weeks refrigerated.