Line the sides and base of an 18 cm square cake tin.
In a medium saucepan, gently heat the condensed milk and butter on low heat until melted. Stir continuously to stop the mixture from sticking to the bottom of the pan.
While the mixture is warm, stir in the chocolate until completely melted. Immediately pour the fudge into the cake tin.
Smooth the top and refrigerate for 2-4 hours until firm.