In a medium saucepan, bring sugar and ½ cup (120 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 high speed until stiff peaks form. With mixer running, pour hot sugar syrup into egg whites in a slow, steady stream. Beat until bowl cools to the touch and frosting reaches a spreadable consistency, about 10 minutes. Use immediately.
Recipe by Bake from Scratch at https://staging.bakefromscratch.com/one-layer-gingerbread-cake/