Preheat your oven to 170C (325F) Gas 3. This is based on fan oven, so if you have an electric one, just make the temperature slightly higher (about 180-190C)
Chop or break your chocolate into small pieces. Put the chocolate and butter in a heatproof bowl over a saucepan of simmering water (do not let the base of the bowl touch the water). Leave until melted and smooth. Alternatively melt it slowly in the microwave.
Remove from the heat and add sugar stirring well until incorporated.
Add the flour and stir well.
Stir in the eggs and mix until thick and smooth. Make sure the mixture is not too hot before adding the eggs, otherwise they can curdle.
Add the coffee extract (if using).
Spoon the mixture into the prepared baking tray and bake in the preheated oven for about 30-35 minutes or until flaky on the top but still soft in the centre. Be careful not to overbake it, otherwise the edges will become hard and crunchy.
Leave to cool in the tin on a wire rack, before cutting into triangles, decorating with icing and peppermint candy canes or stripey drinking straws.