Line 1 large or 2 regular sized baking trays with parchment or greaseproof paper.

Put the flour, sugar and salt in the bowl of a food processor and whizz briefly to combine.

Add the butter and process again until the mixture resembles breadcrumbs.

Add the chopped chocolate and hazelnuts, plus the vanilla extract if using. Process on a LOW speed (so the chocolate doesn't break up too much or melt) until a dough starts to form. Don't be tempted to add water: keep slowly processing and the mixture will start to come together.

Tip: if you don't mind very small pieces of chocolate, you can continue to process until the dough becomes a ball. But if you want to keep some bigger pieces, it's better to finish bringing it together by hand, as per the next step.

Put the dough onto a clean worksurface or silicone mat, and briefly knead into a smooth ball. Divide in two.

Roll each piece into a cylinder 3 - 4 cm in diameter, depending how big you'd like the diameter of the biscuits, and flatten the ends.

Wrap each cylinder in a separate piece of cling film, twisting the ends to close (like a Christmas cracker). Holding the ends of the cling film, roll each cylinder up and down the work surface a few times to get it smoother.

Put the dough cylinders in the fridge for 30 minutes to firm up.

- If you remember to do it, turning a few times will prevent a flat underside

- Can be chilled for longer or frozen if needed.

Meanwhile, preheat the oven to 170° C / 150° C Fan / Gas 3 / 325° F.

Remove one piece of dough from the fridge and unwrap.

Take a sharp knife and cut off slices 5mm - 1cm thick: thinner will give you crisper biscuits, cut thicker for traditional, more tender shortbread.

Transfer to the prepared baking trays, leaving space for them to spread a little. Optional: sprinkle each biscuit with a pinch of sugar.

Repeat with the second piece of dough.

Biscuits can also be frozen at this stage.

Put the trays into the oven and bake until the biscuits are cooked all the way through. This should take 15- 20 minutes, depending on how thick you cut them.

Note that they may still be a little soft until cooler.

Leave the biscuits to cool on the tray for a few minutes then transfer to a wire rack to cool completely and firm up.

When cold, store in an airtight container.

Best eaten within 3 - 4 days.