First, on a tray lined with parchment paper, use a small ice cream scoop to place balls of Nutella, leaving space between each one. Then, freeze them until completely firm.
Next, place the butter in a saucepan over medium heat. Stir occasionally until it melts and turns golden brown with a rich nutty aroma. After that, transfer the browned butter to a bowl and let it cool slightly.
Meanwhile, combine the granulated sugar and brown sugar with the browned butter, then mix until well blended. Next, add the egg, egg yolk, and vanilla extract. Whisk everything together until smooth and fully combined.
In a separate bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt. Then, gradually add the dry ingredients to the butter mixture and mix until a soft cookie dough forms. Finally, fold in the chocolate chips until evenly distributed.
Once the dough is ready, cover it and place it in the refrigerator for 1 hour. This helps the dough firm up and creates a soft, chewy texture.
After chilling, use an ice cream scoop to portion the dough into equal balls. Next, flatten each ball, place a frozen Nutella piece in the center, and cover it completely. Finally, shape each cookie into a smooth ball.
Before baking, arrange the filled cookie dough balls on a parchment-lined tray, leaving space between each one. Then, chill them in the refrigerator for 30 minutes.
Finally, bake the cookies in a preheated oven at 180°C (356°F) for 13–15 minutes. Immediately after baking, add extra chocolate chips on top and sprinkle with flaky salt.