Caramel: Place the heavy cream and butter in a small pot over a low heat so that the butter melts and the cream starts to steam. Meanwhile heat the sugar in a light-coloured pan over a medium heat. Once the sugar starts to melt start to tilt and swirl the pan so that the sugar mixes with the melted caramel and evenly melts. Once the sugar has all melted and has turned from light yellow to amber, turn the heat down to low and pour in the warm cream and butter mixture – this will bubble up so be careful. Stir together until smooth then take off the heat and stir in the salt. Let cool for 5 minutes then stir in the rum. Leave to cool completely. Bake 180C 22-25 min