Preheat the oven to 200°C (180° fan) 400°F gas 6. Grease a baking tray.
Sift the flour, sugar and salt into a mixing bowl. Rub in the butter until the mixture resembles breadcrumbs.
Stir in enough buttermilk (you might not need it all) to form a soft dough. Turn out onto a lightly floured surface and knead lightly until smooth.
Roll or pat out 2.5 cm | 1" thick. Cut into rounds with a floured 5 cm | 2" cutter. Lightly knead together the trimmings and cut out more rounds.
Place on the baking tray. Brush with beaten egg yolk and bake for 15-20 minutes until golden brown.
Allow to cool, cut in half and spread with warm butter to serve.