Combine the sugar and water in a heavy-based saucepan over high heat. Stir just until the sugar dissolves, then bring to the boil and cook without stirring until it becomes a dark caramel. Remove from the heat and carefully pour on the cream, very slowly, as it will bubble up quite violently. Return to the heat and stir until well combined and smooth.
Remove from the heat and allow the caramel to cool to 40°C (105°F). Put the caramel in the bowl of an electric mixer with a whisk attachment and whisk on medium speed, gradually adding the softened butter and salt until combined. Then whisk on high speed until lightly aerated. Allow to cool at room temperature. If not using straight away, refrigerate until required, but bring it back to room temperature before use.
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