Preheat the oven to 180°C / 350°F / Gas mark 4.
Line a baking tray with greaseproof paper then lightly grease them and set aside.
Tip the flour into a food processor.
Cut the butter into small pieces and drop them into the bowl, then blitz until the mixture looks like breadcrumbs.
Add the sugar, vanilla and egg yolks and whizz to a small dough.
With your hands, roll the dough on a lightly floured surface into a sausage shape about 25cm long and 5cm in diameter.
Wrap the roll in cling film and chill for at least 1 hour.
Using a sharp knife, cut the dough into slices, each a generous 5mm thick.
Arrange them on the greased baking trays, spacing the biscuits slightly apart so they have a bit of room to spread as they cook.
Bake for 20 minutes until the biscuits are just turning golden around the edges, then transfer to a wire rack to cool.
Dust generously with icing sugar. The biscuits will keep fresh for up to one week stored in an airtight tin.