Sieve the flour, baking powder, powdered sugar, and vanilla sugar.
Grind walnuts in a blender. Do it in three separated batches, so that they don't transform into walnut butter. If using store-bought ground walnuts, skip this step.
Add ground walnuts to the rest of the dry ingredients. Mix together well.
Cut vegan butter into small pieces and add them to the dry ingredients. Add also soy yoghurt.
Mix with a fork, breaking apart clumps of vegan butter and incorporating them into dry ingredients. When the ingredients are mostly together, mix with your hand, so that both vegan butter and soy yoghurt are well incorporated into the dry ingredients. Don't over knead or the dough will become too warm.
Shape the dough into a ball and wrap it with cling film.
Put the dough in the refrigerator for at least one hour (but you can also keep it there for 3-5 days).
Take half of the dough from the refrigerator and shape a big cylinder from it.
Cut the cylinder into 20 equal discs.
Roll out every disc with your hand into a smaller cylinder then bring its ends together to shape it into a crescent.
Place the crescent cookies on a baking tray lined with baking paper.
Bake on the middle rack at 180 ℃ for 15 min.
While the cookies are baking sift powdered sugar and vanilla sugar for the coating into a small bowl.
Leave the cookies to cool a little bit then dip them into the prepared coating. Take one cookie and put it into a bowl with coating, tossing it around so that all sides of the cookie are coated with powdered sugar and vanilla sugar mixture. Repeat the same with other cookies.
Repeat the same process with the second half of the dough from the refrigerator.