Melt the butter in a medium saucepan with a light colored bottom over medium heat. Once it melts, keep watching it and it will bubble, then foam, then eventually turn a dark golden brown color and have a nutty aroma. Watch carefully and whisk or stirring frequently, because it can go from golden brown to burned very quickly.
1 cup (226g) salted butter
Pour the brown butter into a large mixing bowl so it doesn't continue to cook, then let it cool until warm, not hot.
Add the brown sugar and granulated sugar. Whisk well to combine.
1 1/4 cups (250g) dark brown sugar, 3/4 cup (150g) granulated sugar
Add the eggs, egg yolks, and vanilla. Whisk again until smooth.
2 large eggs, 2 egg yolks, 2 teaspoons pure vanilla extract
Add the flour, baking soda, baking powder, and salt. Stir just until combined.
2 3/4 cups (388g) all-purpose flour, 1 teaspoon baking soda, 1 teaspoon baking powder, 1 teaspoon salt
Stir in the chocolate chunks and chopped candied hazelnuts. Use a medium cookie scoop to scoop balls of dough onto a baking sheet lined with parchment paper. They don't need to be spaced out at this point because they are going into the fridge to chill before baking.
350 grams chopped dark chocolate chunks
Chill the balls of dough for at least 2 hours in the fridge until firm.
Preheat the oven to 350℉.
Line baking sheets with parchment paper. Arrange balls of the chilled dough a few inches apart on the baking sheet so there is room for them to spread while they bake. I like to bake 8 at a time.
Bake for 13-14 minutes until the edges are set but the centers still look slightly underbaked. The cookies will continue to set up while they cool on the baking sheets. Remove from the oven and immediately sprinkle with flaky salt.
Flaky salt