The only gingerbread house icing you'll ever need!
In the bowl of a stand mixer with the whisk attachment, white the egg whites and cream of tartar on medium-high until white and fluffy, about 6-8 minutes.
On low, add the sifted icing sugar 1/4 cup at a time. Turn on high and whip for another 2-3 minutes. The mixture will thicken.
Either transfer to an air tight container or a ziploc/piping bag immediately as the icing sets quickly.