How to Make It
Combine sugar and water in a heavy-bottomed saucepan over medium-high. Cook, stirring constantly, until sugar is dissolved and mixture comes to a boil. As soon as mixture boils, stop stirring, and carefully insert a candy thermometer. Continue cooking until mixture reaches 240°F.
While sugar mixture cooks, beat together egg whites and cream of tartar in bowl of an electric stand mixer on medium speed until soft peaks form. When sugar mixture has reached 240°F, slowly drizzle sugar mixture into egg whites while mixer is running on medium speed. Increase speed to medium-high, and beat until stiff peaks form. Use immediately as desired.