In a food processor, blend together the chocolate and brown sugar until fine.
Mix together the flour, espresso powder, baking soda and salt.
In a separate bowl, blend the eggs and vanilla until the mixture is light and fluffy.
Add half the chocolate mixture and blend at low speed.
Add half the flour mixture and blend.
Repeat blending just until mixed.
Stir in the hazelnuts and mix throughout the dough.
With a roll-pin stretch the dough and cut biscuits of the shapes you prefer.
Bake the cookies for about 25 to 30 minutes or even less or more, it depends on the oven you have: they must results quite dry but of course not burned.
Cool the biscotti completely and then store in a biscuit box or a jar.