In a medium-sized pan, melt the butter on low heat. Increase the heat to medium, and watch the butter as it turns light brown (the butter will turn foamy, so it will be difficult to see. Turn of the heat when you get the strong aroma of brown butter)
Allow the butter to cool for atleast an hour before using it in the cookies.
After the butter has cooled, put it in a large bowl, along with both sugars, and using a whisk or a hand beater, cream until well combined.
Add the egg and vanilla essence and mix well
Sift in the flour, cornstarch, baking soda and salt, and combine using a wooden spoon or a spatula.
Stir in the chopped hazelnuts and chocolate, and form a dough.
Wrap it in cling wrap and refrigerate for atleast 2 hours, or preferably overnight, for best flavour
Once the dough has refrigerated, preheat the oven to 180C
Form roughly 3inch sized balls, place them on a baking tray and bake for 12-15 minutes.
Sprinkle them with sea salt right after they are out of the oven (optional), and let them cool for 10minutes before serving
