In a small pot whisk together eggs, lemon juice, zest and sugar. Cook over a low-medium heat, stirring constantly with a wooden spoon until the mixture thickens (about 10 minutes).
Reduce the heat to low and add butter, stirring to combine. Cook for a further 2 minutes and then remove from heat.
Allow to cool (the curd will thicken even more as it cools) and pour into jars.