Position rack in center of oven and preheat the oven to 325° F. Line a large baking sheet with parchment paper and set aside.

In the bowl of a standing electric mixer fitted with a paddle attachment, combine the flour, cocoa powder, sugar, baking soda, and salt. Mix on low to combine. Turn the mixer off and add the hazelnuts and chocolate morsels and mix on low.

In a small mixing bowl, whisk together the eggs, egg yolk, and vanilla. With the mixer still on low, add the egg mixture to the flour mixture, combining just until the dough comes together. Turn the mixer off.

Using a rubber spatula, scrape the dough out onto a well-floured surface and knead lightly to work in any extra flour mixture from the bottom of the mixing bowl.

Form the dough into a long log, about 3 1/3 inches wide. You will probably have to orient the log on the diagonal to make it fit onto the baking sheet. Using your hands, flatten the top of the dough.

Place the baking sheet in the oven and bake for 45 to 50 minutes. The log should become firm, with cracks in the top, and the dough should be completely dry to the touch.

Using oven mitts or pot holders, remove the log from the oven and cool slightly on the baking sheet.

Reduce the oven temperature to 300° F.

When the log is cool enough to handle, remove it from the baking sheet and cut into 1/2 inch thick slices, on a slight diagonal. You should be able to get about 24 slices.

Lay the slices flat on the parchment-lined baking sheet and bake for an additional 30 to 35 minutes, until the cookies are completely dry and crisp.

Using oven mitts or pot holders, remove the biscotti from the oven. Using a metal spatula, carefully move the biscotti to a wire rack and cool completely.

Store in an airtight container.