350g caster sugar
4 eggs, beaten
2 egg yolks, beaten
100g unsalted butter, diced
Put the orange zest and juice, the sugar and the butter, cut into cubes, into a heatproof bowl set over a pan of simmering water, making sure that the bottom of the basin doesn’t touch the water. Stir with a whisk from time to time until the butter has melted.
Mix the eggs and egg yolk lightly with a fork, then stir into the lemon mixture. Let the curd cook, stirring regularly, for about 10 minutes, until it is thick and custard-like. It should feel heavy on the whisk.
Remove from the heat and stir occasionally as it cools. Pour into spotlessly clean jars and seal. It will keep for a couple of weeks in the refrigerator