For the dough, mix together flour and salt, then add the butter with your hands until you have a crumbly homogeneous mix. Add the water and mix gently with a fork until the dough just comes together. Form the dough into a disk, wrap in plastic and let it chill in the fridge for at least 30 minutes.

Using a rolling pin, roll out the dough on lightly floured surface to a circle, ca. 3mm thick, and transfer it to your prepared 30cm-tart tin. Roll the pin over the top of the tin to trim off the excess dough. Prick the base all over with a fork, then place in the freezer for 30 minutes.

Preheat the oven to 200° C.

For the filling, whisk the egg, milk, vanilla sugar and salt together, and set aside. Cut each rhubarb into long, very thin strands.

Spread the ground hazelnuts evenly over the pastry base, then prep the rhubarb strands into the tart tin in large circles, starting at the rim and working your way to the center of the tart. When finished, spread the egg mix evenly over the tart.

Bake in the lower half of the oven for 35 mins.

If desired, sprinkle the tart with sugar powder when cooled.