Preheat your oven to 340°F/170°C in natural convection/high and low static heat mode. Spread the hazelnut kernels on an oven tray lined with parchment paper and place in the hot oven for around 10 minutes.
Leave the hazelnuts to cool and place in a clean tea towel. Rub with your hands to remove as much skin as possible.
Place the cold hazelnuts in the blender and blend, pausing to avoid damaging the blender. During these pauses, feel free to use a spoon to loosen the hazelnuts from the sides of the blender, to help save time. The hazelnuts will first turn into a powder, then into a purée and finally into a liquid. This takes about ten minutes.
Melt your chocolate in a saucepan over low heat, stirring to prevent it from sticking.
Add the melted chocolate to the blender and blend to combine. Place the spread in a jar and store in a cupboard away from light.
Notes
You can add a pinch of salt to bring out the taste of the chocolate.
I wouldn't recommend reducing the quantities: otherwise you risk your blender/robot running on empty.
While blending the hazelnuts, take breaks with the blender so that it doesn't overheat and break (this has been my experience!).
Only melt your chocolate once your hazelnuts have been roasted and blended, as described in the recipe. Otherwise, it will harden in the pan and you'll end up with a play-dough texture you can't use (more experience!).