Ingredients
300g plain flour
20g cocoa powder
1 tsp baking powder
¼ tsp salt
130g dark brown sugar
50g caster sugar
3 eggs
1 tsp vanilla extract
80g chocolate chips (I like a 50:50 split of milk and dark)
50g hazelnuts or almonds, roughly chopped

Method
Turn the oven to 180c and on a lip lined baking tray, roast the hazelnuts for about 15 minutes, shaking the pan after 10 minutes to make sure they evenly brown. Put to one side to cool.

Line a large baking sheet with either silpat, or baking parchment.

Put all of the ingredients apart from the chocolate chips and nuts into a stand mixer fitted with a dough hook or large mixing bowl. Once almost brought together, add the chocolate and nuts. If you are doing by hand the mixture will get quite stiff but keep mixing until a dough has formed.  Roll the dough into a ball, cover with cling film and refrigerate for an hour.

Preheat oven to 160c.  Lightly flour your work surface, take the ball and cut in half.  Take each half and roll into a log shape.  When rolled, with your hands, lightly flatten the top a little.  Put both logs on to the baking tray apart from each other as they will spread a little. Put into the oven and bake for 35 minutes. 

After the first cook, place the logs on to a wire rack and cool a little and set the lined baking tray aside, ready for the second bake. Reduce the oven to 140c.

Once cool enough to handle, with a breadknife, cut each log into slices 2-3cm thick.  Put on to the baking sheet and bake for another 25 minutes, turning over half way through.  When you touch the biscuits they should have lost their wet look and feel dry.  If they feel at all soft then put in for another 5 minutes before checking again.

Cool on a wire rack.