With a hand mixer, whisk together the butter with both sugars, until pale white and fluffy.
Add the egg and vanilla and whisk until well incorporated.
Sieve together the salt, flour, cocoa powder and baking soda in a separate mixing bowl.
Gradually beat in the flour mixture into the batter - don't over-mix as doing so will result in a harder texture.
Lastly stir in the chocolate and hazelnuts.
Chill the cookie dough in the refrigerator for 15- 20 minutes. Preheat the oven to 180°C.
To ensure these double chocolate cookies are indeed gigantic, weigh cookie dough into balls of 120g-180g each and place on a sheet of parchment. Lightly press on top of the dough to smoothen out.
Bake for 9-11 minutes or until the top is golden brown. Allow to rest for 15-20 minutes, so that they set before taking them off the parchment paper