Break your chocolate into small pieces and place into your slow cooker with the condensed milk and butter. Warm the ingredients on high with the lid off for around 15 minutes until melted.
Stir the mixture with a metal or plastic spoon to ensure thoroughly melted and turn your slow cooker down to low.
Leave the mixture to uncovered on low for about 1hr 30 minutes, stirring every 15 minutes. Meanwhile cover a baking tray (we use a 10" by 8") with greaseproof paper & roughly chop the cranberries.
Once your mixture has cooked for around 1hr 30 minutes add the cranberries and give it all one last stir.
Transfer the mixture to your baking tray and leave to cool before transferring to the fridge for a couple of hours or overnight.
Cut into squares and enjoy!
It's important to leave the fudge uncovered whilst cooking & only use a metal or plastic spoon to stir (not wooden!). Both these things will prevent excess moisture getting into the fudge and thus ruining it.