Melt the butter in a metal bowl over a saucepan of barely simmering water. Lightly beat the eggs and whisk them into the butter. Add the sugar and whisk again until it’s completely combined.
Keep whisking, gradually adding lemon juice and zest. Keep stirring with a wooden spoon for 20 minutes over simmering water, or until thickened. Cool to room temperature – the curd will thicken as it cools.
Sterilise the jars in the dishwasher or using boiling water. Transfer the curd to the jars while it is still warm, but don’t put on the lids until it has cooled completely.