Place the toasted hazelnuts and sugar in a food processor and pulse until a fine, sandy meal is formed.

Transfer the nut mixture to a large mixing bowl and whisk in the flour and salt.

Add the cold, cubed butter to the dry ingredients. Rub the butter into the flour using your fingertips or a pastry blender until a smooth, cohesive dough forms.

Wrap the dough in plastic wrap and refrigerate for 30 minutes to firm up.

Preheat the oven to 160°C (320°F) and line a baking sheet with parchment paper.

Form the dough into small balls, approximately 8-10 grams each (about the size of a marble).

Place the balls on the prepared baking sheet, leaving 2cm of space between them.

Bake for 12-15 minutes or until the cookies are firm to the touch and slightly golden on the bottom.

Remove from the oven and allow to cool completely on a wire rack.

Melt the dark chocolate in a double boiler or in the microwave in 20-second increments until smooth.

Apply a small dollop of melted chocolate to the flat side of one cookie and press the flat side of another cookie against it to create a sandwich.

Set the cookies aside for the chocolate to firm up before serving.