In a bowl, whisk together 2 1/4 cups all-purpose flour, 1 cup hazelnut meal, 1 1/4 teaspoons baking powder, 1 teaspoon baking soda, and 1 1/2 teaspoons kosher salt. Set aside.
In a medium, light-colored saucepan, melt 1 cup unsalted butter, whisking gently as it continues to melt and bubble. After 2 to 3 minutes, you should see brown bits appear at the bottom of the pan — the surface will still be foamy, but the butter will emit a nutty aroma. Remove from heat immediately and continue whisking for another 30 seconds, before pouring into a large bowl. Set the bowl aside on a wire rack to cool slightly.
When the butter has cooled slightly, whisk in 1 1/2 cups dark brown sugar, 1/2 cup granulated sugar, and 1 teaspoon pure vanilla extract. The mixture may look like it will seize, but it will relax after a few seconds of stirring. Add 2 large eggs, one egg at a time, whisking briefly after each addition but only to combine.
Once the eggs have been incorporated into the mixture, swap out your whisk for a rubber spatula. Sprinkle the dry ingredients (from the 1st step) over the surface of the mixture, and use the rubber spatula to fold in the dry ingredients. Be careful not to overmix! At this point, it's okay to have a couple smallish flour streaks left in the mixture. Once mostly blended, stir in 6 ounces coarsely chopped gianduja chocolate and 4 ounces dark chocolate feves. Be sure to bring any stray ingredients up from the bottom of the bowl, and continue mixing until the dough no longer looks dusty. Cover the bowl with plastic wrap and transfer to the refrigerator overnight to allow the dough to rest and chill.
When you're ready to bake the cookies, remove the bowl from the fridge and let sit at room temperature for 30 to 60 minutes to soften the dough. Center a rack in the oven and preheat to 350 (F). Line two baking sheet pans with parchment paper and set aside.
Use a 1-tablespoon cookie dough scoop to measure out 3 tablespoons of dough, using your hands to roll the portioned scoops together to create a bigger ball of dough for one cookie. Repeat until all the dough has been used up. Arrange the dough balls on the parchment-lined baking sheet pans, leaving 3 inches between each cookie. Bake until the tops of each cookie are cracked and lightly golden, around 12 to 15 minutes. The edges of each cookie should be set, but the centers should still be gooey. Transfer the sheet pans to a wire rack to cool for at least 20 minutes, before enjoying immediately.