Place the caster sugar in a large, heavy-based, stainless-steel (so you can see the colour) saucepan with 3 tablespoons of water and bring to the boil over a medium heat, stirring occasionally.
Reduce the heat and simmer, without stirring, for 6-8 minutes until amber in colour.
Remove from the heat and add the brown sugar, butter and cream.
The mixture will bubble, spit and become lumpy but just stir vigorously until it dies down.
Return to the heat, turn it up and allow to bubble for a further 2 minutes, continuing to stir until it forms a smooth thick sauce.
Remove from the heat and serve either warm or at room temperature.
Once cool, this will keep for 2-3 days in an airtight container in the fridge.