Soften the butter in a bowl. Work in the sugar with a wooden spoon. Beat until light.
Sift the flour with a pinch of salt and the chocolate powder. Sift the powder mixture a second time and then stir it into the butter mixture.
Roll the dough into small balls. When flattened these should be biscuit-sized. Place the balls well apart on a greased baking tray and flatten with a wet fork.
Bake for 10 minutes. Leave the biscuits on the baking tray for a further 2 minutes and then lift onto a wire rack to cool.
Glace Icing:
Sift 225 grams of icing sugar into a bowl and mix to a stiff paste with a little water. The water should be added 1 teaspoon at a time.
Stand the bowl in a basin of hot water and stir gently until the icing thins a little.
Coat each biscuit with the icing and leave it to set.
Decoration:
Sift the remaining icing sugar into a bowl.
Mix the egg white with a fork. Beat in enough sifted icing sugar to give a very stiff mixture. Mix in a bit of food colouring. Pipe onto the biscuits.