Cream together the butter and sugar in a bowl. Beat in the egg and whisk in the flour. Roll into a dough ball, cover with clingfilm and chill for an hour. Roll out the dough on a floured surface and use a circle cutter to cut cookies.
Place the cookies on a lined baking sheet.
Bake at 180'c for 10 minutes and leave to cool.
Whisk together the icing sugar, egg white. Add the grey food colouring and add the water, adjusting if the icing gets too runny.
Scrape the icing into a piping bag and pipe onto your cookies.
Spread out the icing with a palette knife.
Dust the top of your cookies with silver edible glitter.