Preheat the oven to 140C/120C Fan/Gas 1. Wash one medium or two small jam jars, then rinse well and put in the oven to sterilise them - or place them in a dishwasher and run through on a very hot cycle.
Beat two eggs, plus two yolks, together in a large bowl. Stir in the caster sugar, orange zest and juice, lemon juice and butter.
Put the bowl over a pan of simmering water, making sure it doesn't touch the water. Stir with a wooden spoon until the mixture is thick enough to coat the back of the spoon. Run your finger down the back of the spoon - if it leaves a path, the curd is ready.
Pour into the jars and cover. Store in the fridge, where it will keep for up to three weeks.