Position rack in center of oven and preheat the oven to 325?F. Line a 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 31/3 inches wide. Using your hands, flatten the top of the dough.
Place the dough log onto the prepared baking sheet 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 12 slices, about 1/2 inch thick, on a slight diagonal.
Lay the slices flat on the parchment-lined baking sheet and bake for an additional 30 to 35 minutes.
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.
Yield: 1 dozen large biscotti