To make the caramel, melt the sugars in a wide, deep frying pan and heat gently until the caramel becomes a dark golden colour. Occasionally tip the pan to make sure one side doesn’t brown before the other. Turn the heat up and carefully stir in the cream (it will start to bubble), followed by the butter and keep stirring until you have a smooth consistency. Take off the heat, pour over the baked shortbread and tilt the tin to spread it until you have an even layer. Leave to cool and set completely.