In a small, heavy saucepan, combine 2⁄3 cup sugar, 1⁄3 cup water, and corn syrup, stirring just until combined. Cook, without stirring, over medium-high heat until a candy thermometer registers 240° (see kitchen tip), approximately 6 minutes. Remove from heat.

In a large bowl, beat egg whites and cream of tartar at high speed until foamy. Gradually add remaining 1⁄3 cup sugar, beating just until soft peaks form. With mixer on medium speed, slowly add hot sugar syrup in a thin steady stream to egg whites. Increase speed to high; beat until stiff peaks form and mixture is thick and glossy. (Do not overbeat.) Use immediately.