Slice the ⅓ cup unsalted butter into pieces and melt it in a light-colored skillet over medium heat stirring or whisking constantly. Once melted, the butter will begin to foam. Continue stirring/whisking for about 5-7 minutes, the butter will become a deep amber color. Remove the browned butter from the heat, and pour it into a bowl. Allow the butter to cool for 30 minutes.
In a large bowl, mix the butter, 1 cup Nutella, ¼ cup brown sugar, ¼ cup granulated white sugar, 2 large eggs, 1 egg yolk, and 2 tablespoons hazelnut liquoruntil combined.
Add the 2 cups all-purpose flour, 1 teaspoon baking soda, and ½ teaspoon kosher salt and stir until just combined.
Stir in the ½ cup chopped hazelnuts.
Wrap the dough in wax paper and refrigerate for at least 2-3 hours.
Preheat the oven to 350°F (180°C) and position an oven rack in the middle of the oven. Scoop 2 tablespoon-sized mounds of dough onto baking sheets about two inches apart.
Bake for 12 - 15 minutes or until the edges are set but the center is still soft. Allow the cookies to cool for a few minutes before transferring them to a wire rack to cool completely.