500g Chelsea Caster Sugar
375ml can evaporated milk
200g dark chocolate, chopped
50g butter, chopped
Sea salt flakes, for sprinkling

Line the base and sides of a 20cm square cake tin with baking paper.
Put sugar and evaporated milk in a saucepan over a low heat and stir until sugar dissolves. Bring to the boil, then reduce to a simmer, being careful mixture doesn't boil over. Stir often for 15-20 minutes until mixture reaches 110-115°C on a sugar thermometer, or when a teaspoon of mixture dropped into cold water forms a soft ball. Remove from heat. Allow to stand for 20 minutes to cool.
Add chocolate and butter and stir until melted and combined. Transfer mixture to bowl and beat for 4-5 minutes until mixture lightens in colour and becomes creamy. Pour into cake tin and allow to cool. Put in the fridge overnight to set.
Remove fudge from tin by lifting edges of baking paper. Cut into squares, sprinkle with sea salt flakes and serve.