The wider the base of your pan the faster the cream will evaporate so you can use a skillet if you wish.
Place all your ingredients in the pan and put on medium heat.
Bring the mixture to a boil, then reduce heat and simmer until reduced by nearly half. stir occassionally and do watch to ensure the mixture doesn't come back to the boil.
This will take 15 to 20 minutes and should be thickened to coat a spoon. Pour the mixture into a medium jam jar which will be heat resistent. The condensed milk will It w thicken as it cools.
Store in the fridge if you aren't using. It will be thick when you take it out, but dip the jar in hot water for a few minutes or microwave for 10 seconds to regain the pourable consistency.
Notes
You don't need the butter, but it does give a nice touch of butterscotch flavouring.[nutrition-label]