Grease an 8inch square tin and line with greaseproof paper.
Put the butter, sugar and evaporated milk in a pan over a low heat and melt gently
Once the sugar has dissolved, add the marshmallows and turn up the heat and boil the fudge for 5-6 minutes.
Take the pan of the heat and add the chopped chocolate. Leave for a minute, then stir the mixture together until everything is melted.
Pour the mixture into the prepared tin and leave to set for a couple of hours.
Once set, remove from the tin and cut in squares.