Preheat oven to 175C (350F). In a blender or food processor, blend sugar and half of the hazelnuts until finely chopped.
In a large bowl, mix the flour, whole hazelnuts, chopped hazelnuts and sugar, baking powder, and salt.
Beat the eggs, hazelnut syrup, and vanilla extract together.
Combine egg and flour mixture with a wooden spoon until a rough dough forms.
On a baking sheet covered with parchment or a silicone mat, turn out the dough into two rough logs. Bake at 175C for 30-40 minutes.
Let cool for 10 minutes. With a serrated knife cut biscotti into 1 cm thick pieces.
Return cut biscotti to baking sheet and bake for 15-20 more minutes at 150C (300F), turning over after half the time. The longer it bakes the harder it becomes. Let cool.
