Preheat oven to 350ºF. Line a large baking tray with parchment paper and set aside.
Bake hazelnuts on a small baking tray until golden and fragrant, about 5-8 minutes. Allow to cool a bit, then place in a clean towel and rub the skins off. Set aside.
Add the bread flour, pastry flour, brown sugar and baking powder to the bowl of a stand mixer and combine using the paddle attachment.
Cut the butter into cubes and add to the flour mixture. Continue to mix to a crumbly texture.
Mix in the dark chocolate, milk chocolate and hazelnuts.
In a separate bowl, lightly whisk together the eggs and vanilla bean paste to break up the eggs.
Add the egg mixture to the flour mixture and mix just until dough has formed.
Divide the dough into two pieces and shape into 24-inch logs on a parchment-lined baking tray. (see it here)
Bake for 20-30 minutes and remove from oven. Lower oven temperature to 300ºF.
Allow the biscotti to cool, then slice at an angle about ¾ inch thick with a large serrated knife.
Return to the oven and allow them to dry for 15 minutes - a little longer if you like a really dry biscotti.
Remove and allow to cool completely before dipping in the dark chocolate.
To make the dip:
Place gianduja in a metal bowl over a pot of simmering water and melt to 45º.
Remove from heat and stir in the dark chocolate until a working temperature of 29º - 30º is reached.
Dip the biscotti and place on a parchment covered baking tray to set.
Notes
The flavour is more pronounced a day or two later, so baking them a couple days ahead is my preference. Store at room temperature in an airtight container.