If you like a smooth creamy luxurious fudge, not at all sugary or hard, and one that is easy to cut into portions without crumbling, then this is the one. Definitely not part of a daily diet. Make for special occasions only.
Spray and line with non-stick baking paper, a 20 x 30 cm, quite deep (at least 4 cm) tin, or 2 smaller tins.
Place all ingredients, except the chocolate and vanilla, in a large heavy based saucepan and stir over medium heat until the butter melts and the sugar dissolves. Bring to the boil and boil gently until it becomes very thick and changes colour to a dark caramel brown paper shade – about 6 minutes, to the soft ball stage, 116°C on a sugar thermometer. Stir often to prevent it catching on the bottom – I find a silicon spatula the best gadget to stir with. Remove from heat and stand until the bubbles subside. Stir in the chocolate and vanilla until melted and smooth.
Pour into the prepared tin and smooth the surface. Cool to room temperature (about 3 hours) then refrigerate until firm. Cut into squares. Store in an airtight container in the fridge for up to 6 weeks – if you’re lucky.