Cream the butter with the sugar until light and fluffy, Gradually add the egg, treacle and vinegar, beating well between each addition. Blend in the pre-mixed dry ingredients. Be sure to scrape down the sides and check under the mixture to incorporate any extra flour. It forms a stiff dough. Take it out of the bowl and wrap it in cling film before placing in the fridge to chill for at least an hour.
Preheat the oven to 190 C / Gas 5. Grease a baking tray or line with baking parchment.
On a well floured surface, roll the mixture to about 5mm thick. Cut out shapes as desired and place biscuits on the prepared baking tray.
Bake for 5 to 15 minutes, depending on the size and thickness of your biscuits.