Preheat oven to 350˚F/175˚C then toast hazelnuts on a baking sheet until they are fragrant and golden, about 5-8 minutes. Turn the oven off for now.

Place hazelnuts into a kitchen towel and gather up ends then shake and rub the hazelnuts around in the towel to remove their skins.

Place the hazelnuts into the bowl of a food processor and discard the skins.

Add the sugar to the food processor and pulse until the nuts and sugar reach a sandy texture, about 30-60 seconds.

Add the flour, cornstarch and salt and pulse to combine.

Add the butter in small cubes and the vanilla extract then process everything together to form the cookie dough.

Wrap the dough in parchment paper and chill for 30 minutes.

Form the dough into roughly 60 small balls, about 1 level teaspoon of dough per ball.

Freeze them on a baking tray for 30 minutes. Once they are frozen they can be transferred to a freezer bag and stored in the freezer until ready to bake if you are making the dough ahead of time.

Preheat the oven to 300˚F/150˚C while the dough balls are in the freezer. Line 2 half-sheet pans with parchment paper.

Arrange the dough balls spaced out to fit 30 per baking sheet.

Bake for 12-15 minutes, switching the trays halfway through, until the cookies are domed and golden.

Let the cookies cool on the pans for a few minutes then transfer them to cooling racks and allow them to cool to room temperature before proceeding.

Chop the chocolate into small pieces.

In a heat-safe bowl set over a saucepan of simmering water or in a double-boiler, melt about 3/4 of the chocolate then remove the bowl from the heat and stir in the remaining chocolate until it melts.

Let the chocolate cool for about 10-15 minutes so it starts to set up. It is easier to sandwich the chocolate when it is just starting to set up, otherwise, it is too runny.

Match up similarly sized pairs of cookies. Working quickly because the melted chocolate will set up fast, sandwich a small spoonful of melted chocolate between the pairs of cookies.