Whisk together the grapefruit juice, sugar, melted butter and eggs until they are completely combined.
Pour the liquid into a heavy bottom pan which distributes heat well. Cook on a low heat and stir continuously while cooking.
Do not boil the liquid otherwise you will end up with grapefruit scrambled eggs!
Do not leave the liquid while cooking and always stir, preventing it from sticking to the base of the pan.
Once the liquid thickens to the consistency of thick custard, pour into sterilised jars and secure with a lid.
Use the curd for tarts, fillings or a spread.