Set the slow cooker to low (we used a 6-litre cooker). Tip in the condensed milk, both chocolates, the sugar, vanilla and a pinch of salt. Cook for 1 hr, stirring well with a spatula every 15 mins to combine and scrape off any bits that start to stick to the bowl, until thick and smooth. Oil a 20cm square tin, then line with baking parchment. Pour the fudge mixture into the tin and chill for 4 hrs.

Cut the fudge into 36 squares using a sharp knife. Will keep in an airtight container in the fridge for three days.