Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper to prevent sticking.
In a medium bowl, combine the all-purpose flour, cocoa powder, baking powder, and salt. Whisk them together until well blended, then set aside.
In a large mixing bowl, beat the eggs and granulated sugar together using an electric mixer on medium speed. Whip them for about 2-3 minutes until the mixture turns pale and fluffy.
Stir in the vanilla extract and orange zest (if you choose to use it) to enhance the flavor profile.
Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined. Be careful not to overmix to maintain a tender texture.
Gently fold in the toasted hazelnuts and dark chocolate chips, ensuring they are evenly distributed throughout the dough.
Lightly flour your work surface, then transfer the dough onto it. Shape the dough into two logs, each about 12 inches long and 2 inches wide.
Place the formed logs onto the prepared baking sheet, leaving adequate space (about 3 inches apart) to allow for expansion during baking.
Bake in the preheated oven for 25-30 minutes, or until the logs are firm to the touch and a little cracked on the surface. Once baked, remove them from the oven and let them cool on the baking sheet for about 10 minutes.
Using a serrated knife, carefully slice the logs diagonally into 1/2 inch thick pieces. This will create the biscotti shape.
Turn the sliced pieces cut side up, and return them to the baking sheet for a second bake. Bake for an additional 10-12 minutes, until they are crispy and golden.
Remove the biscotti from the oven and allow them to cool completely on a wire rack before serving to enhance their crunchiness.
Notes
Serve alongside coffee or tea, or package as gifts.