Mix yeast and flour and make a fountain. Add sugar, vanilla sugar, salt, eggs and make a dough by incorporating little by little some of the flour.
Cut the butter into small pieces. Add to the dough and begin kneading, incorporating the remaining flour to obtain a smooth dough. If it sticks, put it in the fridge for 1 hour.
Roll out the dough into a large rectangle and place on a buttered baking sheet. A large rectangular pizza pan will fit nicely in the size.
Prepare the garnish
Melt the butter with the sugar and water and heat to boiling point.
At this point add the ground and crushed hazelnuts together and let cool. Meanwhile, preheat the oven to 150°.
Assembly
Brush the rolled-out pastry with jam (warm it up a little so that it spreads better).
Spread the hazelnut filling preparation on the previous pastry and bake for 25 to 30 minutes.
About 5 minutes after cooking, cut into 6 cm squares and cut them in half on the diagonal if necessary.
Notes
You can dip a corner into melted dark chocolate, but that's optional.