150ml fresh double cream.
150g butter (I used salted because it's what I had to hand).
150g dark brown soft sugar.
1 teaspoon vanilla extract.
Melt the butter in a heavy-bottom saucepan.
Add the sugar.
Heat the mixture slowly, stirring to avoid the sugar sticking to the bottom.
When it reaches the 'soft ball' stage, take it off the heat.
Wait until it has cooled to luke-warm.
Stir in the cream.
Stir in the vanilla extract.
Pour it into a suitable jar.
 Soft Ball Stage
This means that if you drop a small amount of the molten toffee into a bowl of cold water and then pick it out and roll it between your fingers, it forms a soft squidgy ball. If it's still semi-liquid and sticky you need to heat it some more. If it forms a hard ball that you can't shape with your fingers, you've heated it too much. I'm not quite sure what will happen if you try to make the sauce using hard toffee - let me know if you try it! ;)
The recipe makes a couple of portions more than will fit into a 340g Cartmel jar. I keep it in the fridge because of the cream content. It's a slightly different shade to the Cartmel stuff but I reckon it tastes pretty much identical. Perfect! :)