Preheat oven to 350 degrees F (180 degrees C).  To toast hazelnuts: spread on a baking sheet and bake for 15 minutes or until lightly browned and the skins begin to flake.  Remove from the oven and place nuts in a dish towel.   Roll up the towel and let the nuts 'steam' for 5 minutes and then briskly rub the towel (with nuts inside) to remove the skins from the nuts.  Cool and then chop coarsely.   Set aside while you prepare the dough.

Reduce temperature of oven to 300 degree F (150 degrees C). Line a large baking sheet with parchment paper.

In a food processor fitted with a metal blade, combine the coarsely chopped chocolate and brown sugar and process until the chocolate is very fine; set aside.

Sift or whisk together the flour, cocoa, espresso powder, baking soda and salt into a bowl; set aside. In the bowl of your electric mixer combine the eggs and vanilla extract and beat to blend, about 30 seconds.  On low speed mix in the chocolate/sugar and flour mixtures until a stiff dough forms, adding the hazelnuts about half way through mixing.

On a floured surface divide the dough in half. Form each half into a log 12 inches (30 cm) long.  Do this by rolling the dough back and forth into a cylinder shape with floured hands. Transfer the logs to the baking sheet, spacing them well apart (width-wise on the pan), and pat to even the shapes. Bake until almost firm to the touch, about 35 - 40 minutes (logs will spread during baking). Remove from the oven, place on wire rack, and let cool for 10 minutes.

Using a long spatula transfer the logs to a cutting board. Using a serrated knife cut the dough into slices 3/4 inch (2 cm) thick on the diagonal. Arrange the slices cut-side down on the baking sheet. Bake 15 minutes. Turn the slices over and bake until crisp and dry, about 15 minutes longer. Remove from oven and let cool on wire rack. 

from Williams-Sonoma Kitchen Library Series - Cookies & Biscotti

These are wonderful with a white chocolate glaze: Melt 3 ounces (90 grams) of chopped white chocolate in a small metal bowl placed over a saucepan of simmering water. Watch carefully as white chocolate can burn easily. Stir to melt. Dip one end of the biscotti into the chocolate and let it dry, on a parchment-lined baking sheet.

Store in an airtight container for several weeks.

Makes about 30 biscotti