Beat butter and c. sugar for 2 mins. Add egg and treacle and beat to mix. Mix all dry ingreds in another bowl (except the demerara), then beat in butter mix. Cover bowl with clingfilm and leave in fridge for an hour.
Preheat oven to 180/gas4. Roll spoonfuls of mix in demerara and place in parchment lined trays. Flatten slightly. Bake 18-20 mins. Cool on rack.