Melt the dark chocolate in a bain marie and set aside until it reaches room temperature.
Mix the soft butter with the sugar until you get a smooth and fluffy cream.
Add the egg and mix until smooth.
Gradually add the melted chocolate and vanilla then mix gently.
Incorporate the flour in batches and when the dough becomes too hard to work, stop mixing.
Add enough flour for a firm dough, though soft and not sticky.
Wrap the dough in cling film and refrigerate for an hour.
Preheat the oven to medium heat. Line a large baking tray with baking paper.
Take the dough out of the fridge and roll it out into a 0.3cm (0.12 inch) thin sheet. Cut out circles with a cutter transfer to the tray.
Bake the cookies for 8-10 minutes then leave them on a cooling rack to cool.