In a medium skillet over medium heat, melt the unsalted butter. Stir frequently until it turns golden brown and smells nutty, about 5-7 minutes. Let cool slightly.
In a large mixing bowl, whisk together the all-purpose flour, sea salt, and baking soda. Set aside.
In another bowl, combine the browned butter with brown sugar and granulated sugar. Whisk until well combined, then mix in the eggs and vanilla extract until smooth.
Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Gently fold in the chocolate chips and chopped hazelnuts until evenly distributed.
Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper and spread the cookie dough evenly into the pan.
Bake for 25-30 minutes, or until the edges are golden and a toothpick comes out with a few moist crumbs.
Sprinkle with sea salt, let cool in the pan for a few minutes, then cool completely before lifting out.
Cut into squares or rectangles, and enjoy!