Grease an 18cm square cake tin with butter.
Put the Shaken Udder milkshake, sugar and butter in a heavy-based saucepan. Heat slowly, stirring all the time, until the sugar has dissolved and the butter melted.
Bring to the boil and boil for 15-20 minutes, stirring all the time.
When the mixture reaches the soft-ball stage (115°C on a sugar thermometer), remove from the heat and leave to cool for 5 minutes.
Beat the mixture with a spoon for a few minutes until it starts to thicken and the gloss disappears.
Pour the fudge mixture into the prepared tin and leave it to set at room temperature.