In a mixing bowl, beat room temperature butter with light brown sugar and granulated sugar until the mixture is light and fluffy, which should take about 3-5 minutes.

Next, incorporate the egg and vanilla extract into the bowl, mixing until everything is nicely combined and smooth.

Then, sift together flour, cocoa powder, baking soda, baking powder, and salt. Gradually mix these dry ingredients into your wet mixture on low speed until just blended.

Fold in 3/4 of the chocolate chunks and hazelnuts, making sure they're evenly distributed while reserving the remaining bits for topping.

Scoop the dough into 2 cm balls and place them on a tray. Press the reserved nuts and chocolate into the tops of each ball, cover the tray, and refrigerate for at least 1 hour to enhance flavor and texture.

Preheat your oven to 180°C (350°F). Bake the cookies on lined trays for 9-11 minutes, until the edges are set but the centers remain soft and slightly gooey.

Remove the cookies from the oven and gently shape them if they’re uneven. Sprinkle with flaked sea salt and allow them to cool for 10 minutes before transferring to a wire rack.