In a small saucepan, combine water, corn syrup, and 2/3 cup sugar. Place over medium-high heat and cook until boiling. Cook, stirring, until mixture reaches 240° F on a candy thermometer. Immediately remove from heat.

While the saucepan on the stove top is cooking, beat the egg whites. In the bowl of an electric mixer fitted with the whisk attachment, beat egg whites and cream of tartar together until light and frothy. With the mixer running, slowly pour in remaining 2 tablespoons sugar. Beat until soft peaks form. Let sit until saucepan ingredients are ready.

With the mixer on low speed, drizzle the hot syrup from the saucepan very slowly into the egg whites mixture. Increase mixer speed to high and continue beating for about 7 to 9 minutes, until mixture is stiff and glossy. Add vanilla and beat on high just a bit more to combine completely.

Use in your favorite frosting, as a cookie sandwich spread, or over scoops of ice cream!