Lightly oil an 8in / 20cm square pan and line it with parchment paper. Set a slow cooker to low.
Put 375g of sweetened condensed milk, 250g of milk chocolate, 250g of dark chocolate, 100g of light brown sugar, 1 teaspoon of vanilla extract, and 1 pinch of salt in the slow cooker.
Cook for 1 hour, stirring every 15 minutes, until thick and smooth.
Pour into the pan and chill for 4 hours, until set. Cut into 36 squares.
