Put the butter and sugar in the bowl of a food processor and blitz until well combined. Add the flour and pulse briefly until the mixture resembles breadcrumbs.
Tip out the mixture onto a work surface and gently bring it together, then knead it into a dough. Once the dough is formed, wrap it in cling film and chill for 30 minutes. Preheat the oven to 200°C/180°C fan/400°F/Gas 6.
Once the dough has chilled, lightly flour the work surface and roll out the dough until it is about 3mm thick (about the thickness of a £1 coin). Use the 6cm fluted cutter to stamp out 16 biscuits, re-rolling the trimmings as necessary.
Place the biscuits on the lined baking sheet and chill for another 5 minutes, then use a fork to prick a few lines of holes in the centre of each. Sprinkle the biscuits with a little caster sugar and bake for 10–12 minutes, or until lightly golden.
Remove the biscuits from the oven and allow to cool on the baking sheet for 10 minutes. Then, transfer the biscuits to a wire rack to cool completely. The biscuits will keep in a sealed container for up to 1 week.