Meanwhile, prepare the caramel. In a large non-stick frying pan, heat the condensed milk, golden syrup, butter and sugar until the butter has melted and the mixture begins to boil. Stir constantly to keep it from burning. Turn the mixture down to a simmer and cook for 5-10 minutes or until the colour starts to become golden brown. Remove from the heat and transfer to a heatproof bowl to cool and thicken slightly.