In a bowl, mix flour together with 125g of sugar. Add the egg yolk, butter and 2 tbsp cold water.
Knead the dough by hand until smooth. Wrap the dough in transparent film and refrigerate it for 30 minutes.
For the filling, add blackberries, blackcurrant syrup, maple syrup, 75g sugar and cinnamon in a sauce pan and bring to boil, stirring occasionally. Let it simmer for about 20 minutes.
Preheat the over to 200° C and butter your tart pan.
Roll out 2/3 of the pastry dough and gently press the pastry into the bottom of the tart pan. Prick the dough base several times with a fork. Then cover with the grounded hazelnuts.
Using the remaining pastry dough, cut the pastry into strips to create a lattice top.
Pour blackberry mixture into the tart form and cover it with pastry cut.
Whisk together the egg yolk and cream and brush the pastry cut.
Bake the tart for about 30 minutes until pastry is golden brown.