This is a great master recipe for all things gingerbread. As a thicker icing it works perfectly for assembling a gingerbread house, and when thinned to a pipeable consistency works well for decorating.
pastry bag, round pastry tip for assembly (such as a number 10 or 12), small pastry tip for decorating (such as a number 2 or 3)
Combine the sugar, egg whites, water, vanilla, vinegar and salt in the bowl of a stand mixer fitted with the whisk attachment on low speed to form a smooth paste, about 1 minute. The mixture will be thick and resemble toothpaste. Add an additional 1/2 teaspoon water if needed to smooth out the icing.
Increase the speed to medium-high and whip until fluffy and white, 2 to 3 minutes. When ready, this stiff royal icing should hold its shape and stick to your finger when touched, similar to the consistency of toothpaste. If it isn't at all sticky, add an additional half teaspoon of water and re-whip. Use this stiff icing for assembling a gingerbread house. (Make a double batch if making an advanced gingerbread house mansion.)
To thin for decorating, stir a few drops of water at a time (about 1 teaspoon total) into the icing until it is a thinner consistency that still holds its shape.
To tint the icing, stir in a few drops of gel food coloring until you reach the desired color. (As the tinted royal icing sits, the color will get a shade or two darker.)
If not using right away, cover with plastic so the icing doesn't dry out.