In a medium bowl, whisk the flour, salt, baking soda and baking powder together.: The aroma here is faintly toasted and flour forward, and whisking blends the all purpose flour and leaveners so the lift is even throughout the dough. You should notice a uniform pale mixture with no streaks of baking powder or soda, which is important because clumps of leavener will cause isolated bubbles in the cookies. If you skip thorough whisking you risk uneven rise, so take a moment to stir until homogeneous; a common mistake is to add the dry mix without sifting, which can yield pockets of flavorless dry flour in finished cookies.
In the bowl of the stand mixer fitted with a paddle attachment, beat the butter on medium speed for 1 minute, until smooth. Add the sugar and brown sugar and beat for 2 minutes, until well blended. Beat in the vanilla.: At this stage the kitchen fills with a rich, sweet scent as the butter aerates and the sugars begin to dissolve. The texture should change from glossy butter to a lighter, creamier mixture, indicating air incorporation that will contribute to a tender crumb. You will see tiny air bubbles and the color will pale slightly. Avoid overbeating, which can collapse the structure and lead to flat cookies; if you notice the mixture getting too soft, chill briefly before proceeding.
Add in the eggs, one at a time, beating for a minute after each egg goes in. Reduce the speed to low and add in the flour mixture in 4-5 additions, mixing only until each addition is just incorporated (about 5 seconds for each addition - don't over-mix!): When the eggs are added the batter becomes more fluid and glossy, and beating after each addition ensures the emulsion stays stable, preventing separation. Adding the dry mix in small batches prevents gluten from developing excessively, which keeps the cookies tender. You should stop mixing as soon as you see no streaks of flour; little dry specs are okay because continued mixing will toughen the dough. A typical error is to rush and dump all the flour in at once, producing a dense, rubbery cookie.
Still on low speed, mix in the chocolate chips and the hazelnut (or almond) flour. Refrigerate dough for 2 hours or up to 3 days. If you are planning to freeze a portion - you can scoop out 1 1/2-inch rounds of dough to freeze.: As you fold in the chocolate and hazelnut flour the dough takes on a rustic, studded appearance, and the nut flour subtly changes texture to a slightly grainier, fragrant mix. Chilling firms the fats, allowing flavors to meld and preventing over spread during baking, which leads to thicker, chewier centers. If you choose to freeze scoops now, make sure they are well separated on a tray so they do not fuse. A frequent slip up is skipping the chill, which often results in overly thin cookies that brown too quickly at the edges.
Preheat oven to 350 F with rack centered. Line two baking sheets with parchment paper. Scoop out 1 1/2" rounds of dough onto baking sheet, about 2-inches apart.: Preheating brings the oven to an even temperature so the first few minutes of baking set structure properly, and centering the rack ensures even heat circulation. The dough scoops should glisten slightly and hold their shape; you will hear a soft thump when placing the tray in the oven. Proper spacing prevents cookies from joining as they spread, and parchment aids in even browning and easy transfer. One mistake is overcrowding the sheet, which leads to misshapen, unevenly baked cookies.
Bake the cookies one sheet at a time for 8 minutes, and then, using a spatula, gently press each mound down just a little; rotate the baking sheet when returning to oven. Bake for another 7 minutes, or so, until the cookies are pale brown. They'll still be slightly soft in the center, but that's fine - they'll firm up as they cool. Transfer to rack to cool. Repeat with remainder of dough, always using a cool baking sheet.: During the first bake the edges set and the centers remain soft, the surface will look matte and slightly puffed. Pressing the mounds gently encourages an even thickness so the cookies finish with consistent chew. After the second bake you want to see pale brown edges and slightly soft centers, which indicates they are not overbaked; they will continue to cook on the hot sheet once removed, leading to the perfect tender middle. Using a cool baking sheet between batches prevents premature melting of the next tray, which can cause spreading and thin cookies. A common error is leaving them in too long until deeply brown, which sacrifices the tender center for crispness.