Line the base and sides of a 20cm square cake tin with baking paper. Add the caster sugar to a pan and put over a medium heat and leave until melted. Turn up and continue cooking until caramelised to a dark, coppery brown. Meanwhile, put the brown sugar, butter, cream, glucose and liquorice into a separate small pan and bring to a simmer, stirring to dissolve the sugar and liquorice. Pour the cream mix carefully into the caramelised sugar in two additions then keep cooking until the caramel reaches 125C on a sugar thermometer. Immediately remove from the heat and pour into the prepared tin. Leave for at least 4 hours to fully set. Once cooled, use a sharp knife to cut into squares.