Place all ingredients in the top of a double boiler and cook over gentle heat, stirring constantly with a wooden spoon, until the mixture thickens. To test if mixture is thick enough, remove spoon and draw a finger down the back of the spoon. If the curd is cooked, the mixture on the spoon will remain in place. If still runny, then cook further until successfully tested.
To cook in the microwave, place all ingredients in a large microwave-safe bowl and stir (do not beat) with a balloon whisk. Cook on HIGH for 2 minutes, then remove and stir with whisk. Continue cooking for no more than 1 minutes at a time until mixture is starting to thicken, always stirring in between. As mixture thickens, cook in 30 second bursts to prevent over- cooking.