Preheat oven to 220 degrees C. Line a baking tray with baking paper.
Place flour and baking powder in a mixing bowl then rub in butter until the mixture resembles breadcrumbs. Stir in the sugar.
In a small bowl or measuring jug, use a fork to mix the egg, milk and treacle until well combined. Add to flour mixture to form a wet dough.
Turn out onto a lightly floured surface, sprinkling more flour on top if needed to form a workable dough. Roll out to about 2 cm thick and stamp out rounds with a 5cm pastry cutter, kneading the remaining dough together to cut out more. Place on lined baking tray and bake for about 8-10 minutes.
Sprinkle with caster sugar while still warm, then set aside to cool.