Butter a 19.5cm x 9.5cm x 7cm sized loaf tin. Line with lightly oiled non-stick baking parchment that comes up the sides and ends of the tin.
In a saucepan, bring the cream, rum and butter to simmering, then remove from the heat.
In a separate pan, gently melt the sugar and golden syrup over a low heat until the sugar has dissolved. Do not stir – gently swirl the pan instead. Once the sugar has melted, raise the heat and cook until the temperature reaches 155°C. Turn off the heat and immediately add the cream, then combine using a whisk. Place back on the heat and cook until the temperature reaches 127°C.
Pour into the lined tin. Leave to cool for 15 minutes, then sprinkle with the sea salt. Leave to set at room temperature overnight. Cut into pieces using a lightly oiled sharp knife and wrap each one individually in greaseproof paper.