First of all, turn on the oven and set the temperature to 190°C. Then stir together 100g butter and 50g caster sugar in a bowl until you have a smooth, light coloured mixture. Next, add 150g plain flour and a pinch of salt. Stir thoroughly again. Once you've done that, roll out the mixture to a thickness of 1cm and cut into squares. Finally, put a bit more sugar on top of the biscuits and bake in the oven for 15-20 minutes.