Preheat the oven at 175 degrees Celsius.
Cut the butter in small pieces and place it in a bowl.
Add the flour, sugar, vanilla sugar, and 2 egg yellows, mix by hand the ingredients so it becomes a dough.
Divide the dough in two, and press out the dough on 2 pie forms so the shells cover the forms.
Prick the bottoms with a fork and place the pie shells in the middle of the oven for 20 minutes.
Take out the pie shells from the oven.
Peel and cut the rhubarb stalks in small pieces.
Sprinkle the pie shells with the hazelnuts, and place the rhubarb on top.
Mix the eggs for the filling together with sugar and the heavy cream, and pour it on top of the rhubarb.
Place the pies in the oven for 40 minutes.
Take out the pies from the oven.
Mix the egg whites and the sugar to a hard foam and place evenly on top of the pies using a pastry bag.
Place the pies in the oven for a few minutes until the meringue becomes brown on the surface.