In large bowl beat the butter and the sugar together until smooth and fluffy.
Add the vanilla extract and stir well.
Stir in the cocoa powder and the flour. (Mixture will be crumbly.)
Gently press until the dough will combine. Let it chill in the fridge for 20 minutes.
Preheat the oven to 190 ℃. Line large baking sheet with parchment paper.
Gently roll out the dough until it will be half cm thick. Cut it and place on the parchment paper.
Bake in the oven for 10-12 minutes. Let the shortbread cool completely on a wire rack.