Combine together the icing sugar, cocoa, flour and salt in a large mixing bowl until well blended. Rub the butter into this mixture until there are no longer any lumps. Add the egg, chocolate chips and vanilla extract and mix until a stiff dough forms. Gather up and roll into a sausage shape with a diameter of 5cm.
Wrap in nonstick baking paper and chill in the fridge for at least 30 minutes. Grease and line baking trays with nonstick baking paper. Preheat the oven to 180°C.
Use a sharp knife to cut 1cm thick cookies out of the cookie dough. Arrange spaced out on the lined baking trays and bake for 8-10 minutes until just set. Remove and allow to cool for 5-10 minuets on the trays before removing carefully to a wire rack to cool completely.
Stack and arrange on top of the bar of chocolate when ready to serve.
Freezing and defrosting guidelines
Bake according to the recipe but stop before adding decoration or icing if called for. Allow to cool completely. Wrap well in clingfilm and tin foil (to prevent freezer burn) and freeze for up to 3 months.
Loosen the tin foil and clingfilm but leave to defrost in packaging at room temperature for several hours before serving. If you wish to serve the baked goods warm, place them on a baking sheet for 10 minutes in a moderate oven.
If you wish to add icing or decoration, add them after the baked goods have fully defrosted and are at room temperature.