To make Royal Icing: Beat together 6 tbsp egg whites with 1 tsp vanilla until egg whites are frothy. Gradually mix in 4 cups sifted icing sugar until all incorporated. Turn speed up to high and beat until mixture forms stiff, glossy peaks, about 5 minutes or so. If using food colouring add it and mix. Pour into pastry bags or a ziplock type plastic bag, clipping one of the corners.