Preheat your oven to 180°C and line a baking tray with baking paper.
Cream the butter and treacle sugar together until combined. Mix in the vanilla extract.
Add the flour, coconut, baking powder, salt and cocoa and mix until everything has combined and you have a soft dough. Wrap the dough in plastic wrap and refrigerate for 30 minutes.
Roll small balls of the dough, about the size of a large marble, and arrange them on the tray. Press each ball down lightly to make little discs that are roughly 5mm thick. Bake for 12 minutes until starting to crack a little on top. Transfer to a wire rack and leave to cool completely.
Melt the dark chocolate in a small bowl. Sandwich two cooled cookies together with a teaspoon of melted chocolate. Leave to set. Store the cookies in an airtight container.