Sterilise the jam jars - first wash the jars in soapy water and rinse in clean warm water. Preheat the oven to 140C/275F/Gas 1. Dry the jars, upside down, on a rack for 30 minutes.
Whisk the eggs and egg yolks in a large heatproof bowl until well combined. Add the sugar and stir in the lemon zest and juice. Add the butter and set the bowl over a saucepan of very gently simmering water, making sure the bowl doesn’t touch the water.
Stir the mixture with a wooden spoon for five minutes until the butter melts, then use a whisk to continuously stir the ingredients and cook for 10-12 minutes. The lemon curd should the consistency of custard and leaves a light trail when the whisk is lifted. (The curd will continue to thicken in the jars)
Pour the hot lemon curd into warmed, sterilised jars and leave to cool. Cover the curd with a disc of waxed paper or baking parchment and seal with a lid. Keep in the fridge and use within two weeks.