Position one rack in the middle of the oven, and preheat the oven to 175ºC. Line the cookie sheet with parchment paper.
In a bowl with an electric mixer, cream the butter with the sugar for 1 minute or until a sandy mixture forms. Add the eggs and vanilla and mix until fluffy and lighter in color, about 1 minute more.
In a medium mixing bowl, add the flour, cocoa powder, coffee powder, baking soda, and cinnamon and whisk together. Add to the butter-and-egg mixture in 2 additions, and mix only until combined.
Fold the hazelnuts and chocolate chips into the dough, and mix until evenly distributed. The dough will be thick and hard to stir. If it is too sticky, chill briefly.
Divide the dough into 2 equal pieces. On a lightly floured surface, roll each piece of dough into a log about 5 cm wide by 35 cm long by 4 cm high. Place the logs on the prepared sheet pan with several inches between them.
In a small bowl, mix 1 egg with 1 teaspoon of water to make an egg wash. Brush each log with egg wash, coating them evenly on the top and sides.
Bake for 27 minutes, rotating the cookie sheet from front to back halfway through baking, until very lightly browned and somewhat firm. Remove from the oven and reduce the oven temperature to 150ºC.
Cool the logs for 20 minutes, then place them on a cutting board. With a serrated knife, slice each log on a slight angle into 2 cm thick pieces, keeping them in a row. Slide the row of biscotti together, lift and place back on the cookie sheet, then separate the slices, leave 1 cm of space between each one.
Bake again for 9 to 12 minutes, rotating once during baking, until the biscotti feel slightly firm.
