Preheat the oven to 175 C. Spread the hazelnuts on an ungreased baking tray and bake for 5 to 8 minutes, until fragrant and very lightly browned. Set aside to cool for 10 minutes. Chop them coarsely and set aside (this takes a while so that's why the prep time for this recipe is longer, but I find a food processor chops the nuts up too finely).
When you're done chopping, preheat the oven to 175 C again.
Sift the flour, cocoa, baking powder and salt (if using). Add the brown sugar, then the butter and rub it in with your fingers till the mixture resembles coarse breadcrumbs and no dry flour remains. Stir in the chopped hazelnuts.
In a small bowl, combine the eggs, oil and vanilla. Whisk to combine, then pour in the melted chocolate and whisk again till fully incorporated. (see photos below)
Pour the liquid mixture into the flour mixture and gently stir together with a spatula to combine into a rough dough, so that the dry ingredients are moistened. Tip onto a lightly floured surface or silicone baking mat (easiest for clean-up and preventing sticking).
Gently knead the dough 8 to 10 times until smooth, a little sticky and quite soft. Cut into two equal halves.
Place another silicone mat or sheet of non-stick baking paper on the counter and place one half of the dough on it. Using extra flour to sprinkle on, shape each portion of dough into a slab, about 8x4 inches. You don't need to be too precise here.
Bake each slab of dough for 25 to 30 minutes until you see small cracks on top and the top and sides look set. It's hard to notice browning since the dough is already dark. Let it cool till you can safely touch it (and meanwhile bake the second log if you're baking in batches).
Once the slabs have cooled for about 15 to 20 minutes, use a long serrated knife to slice each one into roughly 1 inch thick pieces (with the longer side of the slab facing you). Each slab will give you 8 to 10 pieces.
Turn each piece around so that the cut side is facing up (the side where you'll see the hazelnuts peeking out). You may not be able to do this with the edge pieces, just place them flat on the tray. The dough is still soft at this stage so be careful while turning the pieces around as they might break.
Bake again for 10 minutes, then turn the pieces over and bake again for 10 minutes so that both sides are cooked through. The biscotti will still feel a little soft at this stage. Let them cool at room temperature completely and they will harden on their own. If at all you feel they're still a bit soft in the center (some of the larger pieces possibly), bake for another 2 to 3 minutes as needed.
Once completely cooled, store in an airtight tin at room temperature for a week to 10 days. I find that they do soften a bit once covered, but still have a nice bite. You can also refrigerate or freeze for longer. Enjoy with hot coffee!