In a small saucepan, bring sugar and ¼ cup (60 grams) water to a boil over high heat. Reduce heat to medium-high; cook until a candy thermometer registers 248°F (120°C).

Meanwhile, in the bowl of a stand mixer fitted with the whisk attachment, beat egg whites and cream of tartar at medium-high speed until stiff peaks form.

With mixer at medium speed, pour hot sugar mixture into egg white mixture in a slow, steady stream, avoiding the whisk; beat on medium-high speed until mixture cools to room temperature and is a spreadable consistency, about 10 minutes. Use immediately.