In a large bowl, whisk together flour, hazelnuts, baking powder, baking soda, and salt. Set aside.
In medium sauce pan, melt ½ cup of butter over medium heat. Once the butter has melted, continue to cook. Swirl pan occasionally. Mixture will bubble and foam as it cooks. Continue to cook until butter smells nutty and fragrant. The milk solids in the butter will turn rich brown in color. Remove from heat and pour into a heat-safe bowl. Allow brown butter to cool for 30 minutes.
Pour cooled butter into a mixing bowl. Add brown sugar and granulated sugar. Mix until sugars are moistened. Add remaining 2 Tbsp of softened butter and beat in until combined.
Add egg yolks, egg, and vanilla. Mix until incorporated. Add flour mixture in three batches. Mix on low speed until there are no longer any streaks of flour. Use a sturdy spatula to fold in chopped chocolate. Transfer cookie dough to a large bowl. Cover with plastic wrap and let chill in the fridge for at least 1 hour (up to overnight).
Preheat oven to 350° F. Scoop tablespoon portions of cookie dough. Place cookie balls 2-inches apart on a parchment lined baking sheet. Bake for 12 minutes, rotating pan midway through baking. Immediately sprinkle with Maldon sea salt. Allow to cool in pan for 2 minutes before transferring to wire rack to cool completely.