Royal icing is the classic gingerbread-house-assembly icing and it’s the icing used to decorate all the picturesque sugar cookies you see on Instagram. It can be thinned with a bit of water to get those “floods” of smooth glossy colour, or left a bit thick for precision piping. It dries very hard, so be sure to work quickly and to keep your unused icing in airtight bags or squeeze bottles in the fridge if saving for future use.