First, let's make a flax egg. Add ground flax seeds to a bowl and add water. Let it sit for a few minutes.
In the meantime, add flour, sugar, vanilla sugar, and walnuts to a large bowl. Mix them well.
Add cold butter, cut into small chunks. Start kneading the butter into the flour until you start getting a crumbly texture.
At this point, add a flax egg. Keep kneading until you get a soft ball of dough.
Wrap it up into a wrapping foil (I use reusable wax paper) and set int the fridge for at least 30 minutes.
When the dough is cold, take it out and start shaping the cookies. Each one will weigh approx. 20 g. On a lightly floured surface, roll each of the pieces into a thin snake. Bend it into a moon shape and place it on a baking sheet lined with parchment paper.
Bake cookies at 180°C for about 15 minutes or until they start getting slightly brown on the edges. They might be a bit soft right after you take them out of the oven, but don't worry, they'll harden up once they cool down.
If you want, you can roll them in powdered sugar and serve them with some hot tea or cocoa.
