Preheat the oven to 375°F (190°C). Line baking sheets with parchment paper, and set aside.
Put the brown sugar and granulated sugar in a large bowl and set it aside.
Put the unsalted butter in a pan on medium heat for about 5 minutes until the butter turns golden and the milk solids have browned at the bottom of the pan. Pour the browned butter over the sugar and stir. Scrape all of the browned milk solids off the bottom of the pan and add it to the bowl. Set aside to cool.
In a separate bowl, add the all-purpose flour, baking soda, and fine sea salt and whisk together to combine.
To the browned butter mixture, add the egg and egg yolks one at a time and stir after each addition. Add the vanilla extract and stir again.
Put the flour mixture into the butter mixture and mix together with a spatula or wooden spoon. When most of the flour is mixed in but you still have bits of flour showing, add the chocolate chips and hazelnuts if using, and fold them in only until the flour doesn't show anymore.
Using a cookie scoop (mine holds 4 teaspoons), scoop out the dough onto the baking sheet and sprinkle on a pinch of flaky salt on each dough (optional: top with some chocolate). Bake in the oven for 6-8 minutes. Let cookies cool completely on the baking sheet.