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Put your ready-to-roll icing block in a large bowl and pour over boiling water to cover. After 10-15 mins the icing should be soft. Pour off water, and mix icing in bowl, adding a dribble more hot water until you get a smooth, runny but thickish icing. Divide evenly in 3 and add a little blue colouring to one third, and a little blue and green colouring to another third.
Slice a 20cm square, or a 30 x 20cm traybake cake into roughly equal, portion-sized squares, and sit them on a wire rack above a large tray.
Spoon some icing over each cake to cover, then decorate with sprinkles, sugared almonds, mini marshmallows and small gingerbread men.