Heat the oven to 180C (160C fan)/350F/gas 4.

Spread the hazelnuts on a baking tray and roast for 15-18 minutes, until golden inside when halved.

Remove, leave to cool, then roughly chop half the nuts.

In a large bowl, whisk the sugar, oil and 85g water until smooth and combined.

If you have a used vanilla pod, microwave it for 20 seconds, leave to cool, then break it up and put it (or the vanilla paste/extract) in a food processor with the unchopped cooled hazelnuts, flour, baking powder, bicarb and salt.

Blend to a fine powder – the mix will start to clump as the oils are released from the nuts.

Using a spatula, stir the sugar mixture and chocolate into the dry ingredients, until it all comes together into a dough.

Cover and chill for at least two hours, or overnight (the dough will keep for up to three days in the fridge).

Heat the oven to 200C (180C fan)/390F/gas 6 and line a baking tray with greaseproof paper.

Divide the dough into 70g portions (about the size of a golf ball), then roll into balls and press them into the chopped nuts to give them a crunchy topping.

Arrange on the lined tray about 5cm apart and bake for 12 minutes for soft, fudgy cookies, or for 15 minutes for ones with crispier edges.

Remove from the oven, leave to cool on the tray for a few minutes, then transfer to a rack to cool completely.

Store in an airtight container for up to five days.