Brown the butter: Place butter in a light-colored pan over medium/low heat. Cook, stirring frequently, until melted, then turn heat to medium. Stir constantly (use a spatula and scrape the sides and bottom of the pan) as it foams and sizzles. Once you see an abundance of golden brown specks and the butter has a nutty aroma, immediately remove from heat and pour into a heat-proof bowl. Allow butter to cool completely before proceeding.

Once butter has cooled, add sugars and stir until combined.

Add eggs and vanilla extract, stir well.

In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, baking powder, and salt.

Gradually add the dry ingredients to the butter mixture, stirring until completely combined.

Add chopped hazelnuts and chocolate and stir until well incorporated. Cover bowl with plastic wrap and chill for at least 1 hour and preferably overnight.

When dough has chilled, preheat oven to 350F (175C). If dough has been chilling overnight, pull it out of the fridge to soften (enough to be scoopable) while the oven preheats.

When oven is preheated, scoop dough into 3-Tablespoon-sized balls and place on parchment paper lined baking sheet, spacing at least 2” apart.

Transfer to center rack of 350F (175C) preheated oven, baking 11-12 minutes or until edges are beginning to turn light golden brown. Allow cookies to cool for 5-10 minutes on baking sheet before sprinkling with sea salt (if desired) and carefully transferring to cooling rack to cool completely.