Put the condensed milk, sugar, butter and golden syrup in a medium, heavy-based saucepan and heat gently, stirring, until the sugar has dissolved. Cook over a gentle heat, stirring, for 5 minutes or until the mixture has turned a pale fudge colour.
Leave to cool for 2 minutes, then spoon the toffee over the top of the cooled cakes.
Melt the plain and milk chocolate separately (see
Repeat on the remaining cakes. Leave the chocolate to set before serving.