Step 1

First sterilise the jam jars. Wash the jars in soapy water and rinse in clean warm water. Allow them to drip-dry, upside down, on a rack in the oven set to 140C/120C Fan/Gas Leave them there for at least half an hour while you make the salted caramel.

Step 2

Heat the condensed milk caramel, butter and 8 tbsp of water in a pan and bring to a simmer. Stir until the caramel dissolves. Add the salt and the rum (if using), mix, then leave to cool slightly.

Step 3

Pour into a sterilised jar. This will keep in the fridge for 1 week … if you can make it last that long.