Mix the whole eggs, egg yolks, sugar, salt and zest from the lemons in a small saucepan with a whisk. Start heating at low heat, stirring. Add lemon juice and butter. Stir all the time to the point when the mixture starts to boil and thickens significantly.
Pour the lemon curd into a clean jar and serve with your favourite biscuits, or use it to boost the flavour of other desserts.