In a small bowl, whisk together the all-purpose and hazelnut flours, the baking soda, baking powder, and salt; set aside.

In a stand mixer fitted with the paddle attachment, cream the butter, brown sugar, and vanilla on medium speed until creamy, about 3 minutes.

Add the eggs, one at a time, beating on medium speed until incorporated. Add the flour mixture all at once and mix until just incorporated. Mix in the chopped chocolate and mix until just incorporated.

Line a baking sheet with parchment paper. Scoop the cookie dough using a #16 (¼-cup) portion scoop onto the prepared baking sheet. You can make the cookies larger or smaller, but it will affect the baking time.

Refrigerate the dough balls for at least 30 minutes. If you have the time, they improve if you let them sit for 24 to 36 hours. After they are chilled, you can bake them or freeze the dough balls for baking later.

Preheat the oven to 350°F / 175°C. Line as many baking sheets as you need with parchment paper. (You will need four baking sheets to bake the entire batch.)

Evenly space six chilled cookie balls on each baking sheet and sprinkle each with flaky sea salt (if using). Bake, one sheet at a time, in the middle of the oven for 12 to 15 minutes, until golden brown on the bottom and just set on top.

Allow the cookies to cool for a few minutes on the baking sheets, then move to cooling racks.