Grease and line a 9" square tin.
Melt the butter in a saucepan and mix in the milk. Add the sugar bit by bit and boil for about 4 minutes.
Slowly stir in the condensed milk, stirring so it doesn't stick to the bottom of the pan. Bring to the boil again and boil for about 20 minutes, stirring regularly, until the mixture has become a caramel colour.
Remove pan from heat and beat the mixture until it's nearly set.
Pour into the prepared tin and mark out cutting lines. Leave to cool and set fully, then remove from tin and break along cut lines.