Heat the fresh cream and keep warm. Caramelise the castor sugar over low heat until golden brown, then remove the pot from the heat and add the cold butter in 3 stages.
Add the cream in 4 stages, using a whisk to stir in each small amount. When all the cream is added bring to the boil for 1 minute. Remove from the heat and allow to cool, before transferring to a squeeze bottle for servi