6ozpremium dark chocolate bars (I used Ghirardelli 60%)(chopped into fine pieces to small chunks).
6ozpremium milk chocolate bars chopped into fine pieces to small chunks
Flaky sea salt for sprinklingoptional
Place your butter in a medium-sized saucepan and melt over low heat.
Increase heat to medium (do not walk away from your butter or it will burn, trust me) and swirl occasionally with a wooden spoon as the butter snaps and crackles.
Let the butter snap and pop (while stirring occasionally), and once this begins to slow, begin to stir the mixture constantly.
The butter will be foamy so it will be difficult to tell once it has begun to brown, but once you begin to smell the unmistakably nutty aroma of browned butter, remove from heat (still stirring continuously) and pour into a heat-proof container (If you accidentally burn the butter, which you will know if you did because it will smellburnt, then do not use it or you will have burnt-tasting cookies.).
Allow butter to cool for at least an hour and a half before proceeding.
Once butter has cooled, combine in KitchenAid (or with hand-mixer) with sugars, beating on medium-speed until well combined
Add eggs one at a time, pausing to scrape down the sides and then beating well after each addition.
Stir in vanilla extract.
In separate bowl, whisk together flour, cornstarch, baking soda and salt.
Gradually (with mixer on low-medium speed) add flour mixture to butter mixture, pausing periodically to scrape down sides of bowl (Caution - this dough is thick, you may need to switch over to stirring by hand as my KitchenAid started to struggle).
Stir in hazelnuts and your chocolate chocolate pieces.
Scoop cookie dough into about 3 Tbsp-sized balls (I roll together two leveled scoops from my 1 ½" Tbsp cookie scoop), place in wax-paper lined container and repeat until all dough has been rolled into balls. Cover with lid or clear wrap and refrigerate at overnight (preferably at least 12 hours, for best flavor).
Once dough has refrigerated, preheat oven to 350F and line cookie sheets with parchment paper.
Bake 11-14 minutes (be aware that if you let your cookie dough sit out while the first sheets cook, the cookie dough will approach room temperature and will require less time to cook -- that is why there is so much variability here. Most of my cookies took about 12-13 minutes, but I prefer them a bit under-baked. If you do not like yours under-baked, watch for edges that are just beginning to turn a slightly golden brown).
Remove from oven and lightly sprinkle with flaky sea salt, if using.
Allow cookies to cool at least 10 minutes on cookie sheet before transferring to cooling rack to cool completely.