This is the traditional icing for glazing cookies, piping decorations, or assembling the walls of a gingerbread house. It's light and fluffy; the more you beat it, the stiffer it becomes. For a thinner version, simply thin with a bit of water until you have a consistency that will flow evenly over the cookie. Let it dry to a hard, shiny surface, then you can pipe over it or use food-safe markers to decorate. To color the frosting, we recommend gel paste or powdered colors. Liquid food coloring can dilute the frosting so much it separates and becomes grainy. Be sure to store any frosting you're not using in a tightly covered container; it becomes quite hard as it dries. We make this big batch of frosting, then divide it into smaller amounts to tint different colors.