River Cottage

In a food mixer fitted with the paddle beater, or using a hand-held electric beater, cream the butter and sugar together on medium-high speed until very pale (almost white) and fluffy, scraping the sides down regularly with a spatula.

Lower the speed to medium, and add the salt, egg yolks and any extra flavourings at this stage. Beat for another half a minute, then switch the machine off. Fold the flour into the mixture, using a spatula. You will have a very soft sticky dough.

With floured hands, scrape the dough out of the bowl on to a floured surface. Pat it into a wide, flat disc, using more flour if you need to, then wrap in cling film and refrigerate for about 1 hour, as the dough needs to firm up before you can roll it out.

Preheat the oven to 180°C/Gas Mark 4. Unwrap the dough and roll it out on a floured surface to a thickness of about 3mm. Using a 6cm pastry cutter, cut out about 24 rounds. With a palette knife, carefully transfer them to a non-stick baking sheet, or one lined with baking parchment.

Bake for 7–10 minutes, until just golden around the edges, but pale on top, checking every minute after 7 minutes. The shortbread biscuits will still be soft; they firm up on cooling. As you remove them from the oven, sprinkle generously all over with caster sugar. Leave on the baking sheet for a minute or two, then carefully transfer to a wire rack to cool.

P.S. These delicate biscuits will keep for a couple of days in an airtight container.