Sift the cocoa powder into a bowl.
Measure out three tablespoons of margarine and place them in a small saucepan. Melt the margarine.
Add to cocoa powder and whisk well.
Combine remaining margarine, sugar and vanilla sugar and, with a hand-held mixer, beat until foamy, for about 3 minutes.
Add this mixture and the soy milk to the black cocoa mixture.
Add flour, baking powder and salt.
With your hands, knead the dough until smooth.
Shape the dough into a disk and wrap in plastic.
Refridgerate for 1 hour.
Preheat the oven to 160°C.
Roll out the dough between two layers of plastic wrap.
It should be around 0.2 to 0.4 cm thick.
Cut out cookies with a round cookie cutter (6 cm in diameter).
Gather the remaining dough, roll it out, and cut out more cookies until there is no more dough to roll out.
Place on a baking sheet lined with parchment paper.
Bake for 7-9 minutes depending on the thickness.