These are awesomely delicious and we should make them again. They freeze well. Makes quite a lot, would be excellent for Christmas gift biscuits.
Combine the butter, sugar and treacle in a small saucepan over low heat. Cook, stirring, for 3 minutes or until butter melts and mixture is smooth. Remove from heat and set aside for 5 minutes to cool slightly. Add to the flour mixture with the egg and stir until well combined. Cover with plastic wrap and place in the fridge for 2 hours or until firm.
Place the demerara sugar on a large plate. Roll tablespoonsful of dough into balls and roll in sugar to coat. Place on the lined trays and press to flatten to a 4cm disc. Bake in preheated oven, swapping trays halfway through cooking, for 12-15 minutes or until golden brown. Remove from oven and set aside on trays to cool completely.