Spread the hazelnuts on a baking sheet and toast them in the oven at 180C until golden brown and fragrant. Allow them to cool before proceeding.

Using a food processor or blender, grind the toasted hazelnuts until they reach a fine, sandy texture resembling hazelnut flour.

In a mixing bowl, cream together by hand softened butter, salt and sugar until light and fluffy. Gradually add the ground hazelnuts and sifted flour, mixing until a smooth dough forms. Lastly, add the egg white and combine.

Let the dough rest in the fridge overnight before shaping.

The following day, it's time to shape. Pinch off small pieces of dough, weighing out every single one at 8g roughly, and roll them into small balls.

Place the balls on a baking sheet lined with parchment paper, leaving space between each cookie. Let them rest in the fridge for an hour.

Preheat the oven to 160°C and bake the cookies for 20 minutes or until lightly golden around the edges. Allow them to cool completely before filling.

In a small saucepan, add the chopped dark chocolate and let it melt until it is completely smooth. Move it into a piping bag and let it cool slightly.

Pipe a small dollop of melted chocolate onto the flat side of one cookie and wait a few minutes for the chocolate to settle before you sandwich it with another, pressing gently to adhere.