1. Place the sugar in a medium-sized, heavy-bottomed saucepan and pour the water over it. Microwave the cream until warm then place nearby along with the measured butter and salt. 
2. Put the pan over medium-high heat and whisk until the sugar dissolves. Stop whisking once the sugar starts to boil. Let the sugar boil, swirling the pot occasionally, until it begins to turn light brown. Then watch it very carefully and remove it from the heat as soon as it begins to look amber/dark caramel coloured. 
3. Whisk in the butter until smooth, then slowly whisk in the warm cream in a thin stream. The mixture will bubble up (which is why you shouldn’t use a small pot) but keep whisking until smooth. If your caramel clumps up, stir it over gentle heat until it melts. Stir in the salt and pour into a heat-proof jar. Place in the fridge. 
4. To serve: Drizzle the sliced cake with warm caramel sauce and enjoy. Store leftover caramel in the fridge and leftover cake covered at room temperature.