Pre-heat the oven to 200°C (400°F) and line a large flat baking tray with baking paper.
In a large bowl, beat the butter and sugar together, then whisk in the egg.
Sift the flour, cocoa, baking powder and vanilla sugar together and add to the wet ingredients. Stir until combined. If the dough is a little sticky, add a touch more flour.
You should be able to press the dough out with your hands. Work it until it is a long and fairly flat roll around 1cm in thickness (take a look at my video if you're unsure). If you are doubling the recipe, make sure there is space in between your rolls.
Bake for around 15 minutes, checking partway through to make sure they’re not browning too quickly.
Remove from the oven. Let rest for a minute, and then slice into around 10 diagonal strips (make sure you do this when they are still warm as they will just shatter if they have hardened). Enjoy!