Reduce the oven temperature to 150 Deg C (300 Deg F)
In a food processor or a powerful blender, add in the coarsely chopped chocolate and brown sugar and process until the chocolate is very fine.
120 gms Dark Chocolate, Coarsely chopped, 1 Cup (210 gms) Dark Brown Sugar
Then in a large bowl , sift the flour, cocoa powder, coffee powder, baking soda and salt.
1/3 Cup (30 gms) Cocoa Powder, 1 tbsp (4 gms) Instant Coffee powder, 1 tsp (5 gms) Baking Soda, 1/4 tsp Salt, 1 3/4 Cups (210 gms) All purpose Flour/Plain Flour/ Maida
In a bowl of electric stand mixer or with a hand mixer beat the eggs and vanilla extract until combined (around 1 min)
3 Eggs, 1 1/2 tsp Pure Vanilla Extract
Add the chocolate/sugar mixture and combine.
Next add in the flour mixture and beat until it roughly forms a dough, adding in the coarsely chopped hazelnuts in between.
Dust your working surface with flour and with floured hands divide the dough into 2 equal portions.
Form a log shape from each of the 2 portions. (You might need some extra flour if your dough is wet. Do not add too much flour though)
Transfer the log's into a prepared baking sheets lined with parchment paper.
Bake for 35-40 minutes at 150 Deg C until firm to touch.
Remove from the oven, and let it cool down on a wire rack for 10-15 minutes.
Transfer the cooled logs into a cutting board.
Using a serrated or a sharp knife cut into slices 3/4 inch (around 2 cm) thick.
Arrange the cut slices on a baking sheet.
Bake for 10 minutes.
Turn the slices over and bake again for 10 minutes or until crisp and dry.
Remove from oven and let it cool on wire rack.
This will store in an airtight container for several weeks.
One can even glaze it with chocolate.