The icing used for decoration will most likely pop up here and there, so I thought I will just dedicate a whole post to it..
1 egg white
200 g icing sugar
1 tsp vinegar - optional, makes the icing more shiny
2 flat tbsp of food starch - needed only if you want more professional looking cookies, of course healthier without
Beat the egg whites with the sugar and vinegar, adding the sugar gradually. When you have a nice and pure white texture add the food coloring and food starch.
If you use it for decorating with thin lines, it is going to dry in a matter of hours. If you decide to coat a surface with the icing, let it dry overnight before adding further decorations on top or packing it.