Preheat oven to 325°F (160°C). Position rack in the center.
Line a large baking sheet with parchment paper.
Spread hazelnuts on a baking sheet and toast in the oven for about 12-15 minutes, or until skins begin to crack. Wrap nuts in a kitchen towel for a few minutes. Rub hazelnuts together to remove the skin. Let cool and coarsely chop.
Increase the oven temperature to 350°F (175°C).
In a medium bowl, sift or whisk together the dry ingredients (flour, cocoa, baking powder, and salt). Set aside.
In a large mixing bowl or bowl of an electric mixer fitted with a paddle attachment, beat butter and sugar until fluffy.
Beat in the eggs, one at a time.
Add vanilla extract.
Add flour mixture to egg mixture; stir with a wooden spoon until just incorporated (do not over mix).
Fold in chopped hazelnuts and chopped chocolate.
Divide dough into quarters. Place each quarter on the cookie sheet, shaping each into a 15-inch log. Flatten each biscotti log until 1½ inches wide.
Bake for approximately 23- 30 minutes or until firm to the touch. Transfer to a rack and let cool slightly.
Transfer to a cutting board.
Using a serrated knife, slice cookies at an angle about ½-inch thick.
Place the sliced cookies back on the baking sheets, and return them to the oven for about another 20-30 minutes (the longer they stay in the oven, the crisper they get). Turn them over at around the 10-minute mark.
Remove from oven and enjoy.
Replace hazelnuts with walnuts, pecans, or even almonds if more convenient.
For best results, weigh the flour and the cocoa powder.
Use either natural or dutch process cocoa powder.
For a more decadent biscotti, try dipping one end in melted chocolate. We would probably have to change the name to double chocolate biscotti or perhaps triple chocolate biscotti!
Although these cookies are twice baked, feel free to omit the second baking time for a more tender cookie.
Replace the bittersweet chocolate chunks with chocolate chips.