In a large bowl or the bowl of your stand mixer fitted with the whisk attachment, combine the meringue powder and water and whip on high until very foamy, about two minutes. Add the icing sugar and mix on low to combine then whip on medium until thick and very stiff. If needed, add a little more icing sugar to thicken the mixture or a little more water to thin it.
Tint with food colouring if desired, transfer to a piping bag fitted with whatever tip you’d like, and decorate your cookies. Allow the icing to harden before digging in or hanging on your tree.