Grease a 20cm/8in square cake tin with vegetable oil, then 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. Boil the fudge for 5-6 minutes.
Take the pan off the heat and add the chopped chocolate. Leave for one 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 into 25 squares.