Lightly oil a 20cm square cake tin. Place all the ingredients, except the vanilla, into a large heavy-based pan and heat slowly, stirring until the butter melts and sugar dissolves. Bring to boil, stirring until the butter melts and the Molasses Sugar dissolves.
Bring the mixture to the boil then reduce heat to a rapid simmer and cook, stirring occasionally to prevent it catching on the base of the pan for 10-15 minutes or until the sugar thermometer shows that is has reached 117C.
Beat the mixture with a spoon for a few minutes until it starts to thicken and the gloss disappears. Carefully pour into the prepared tin and leave to set at room temperature. Once set, chop into squares and store in a sealed container.