Sift together 500g icing sugar and 450g very finely ground almonds three times in a bowl. Add 2 large egg whites and 5ml (1 tsp) vanilla extract and mix to make a stiff paste. Knead until smooth. Wrap tightly in plastic wrap and store in the fridge for up to one week – if it lasts that long! Serve it as is, dust with cocoa powder or use on cakes.