Put the 175g / 6oz of butter in heavy-based pan and melt over a moderate heat. Add the treacle, vinegar and sugar with 2tablespoons of water. Bring to the boil and boil rapidly, stirring with a wooden spoon to prevent burning. Test a teaspoon of the mixture in cup of cold water; if it hardens immediately, it is ready. Stir in the cream and pour into the prepared pan. When very cold, mark out into squares.