Preheat your oven to 180oc/350 Fahrenheit/gas 4 and line your baking sheet(s) with baking or greaseproof paper (I always use 2 with 6 cookies each but if you only have 1 you can bake them in batches)
Cream the butter and sugar with an electric whisk or in the bowl of a stand mixer.
Add the vanilla and egg and beat until just incorporated.
Add the flour, salt and bicarbonate of soda in batches, mixing thoroughly with a wooden spoon or spatula.
Mix in the chocolate and hazelnuts, making sure they are evenly distributed.
Using a cookie or ice cream scoop (or a dessert spoon if you don't have either) scoop and shape 12 balls and place them on the cookie sheet or baking trays - I'd suggest 6 per tray to avoid making yourself 1 huge cookie mess. If the dough is a bit too soft to handle and shape, stick it in the fridge for a little bit.
Bake in your preheated oven for 9-12 minutes, until they are just beginning to turn golden brown, then leave to cool on the trays for 20-30 minutes before serving.
If you can manage to avoid eating them all at once, they will store in an airtight container for 2 days (I don't know who I'm kidding here, because a batch of these never last 24 hours in my house!)