Put zest and juice in a saucepan, add the butter and sugar and stir until the butter has melted and the sugar is beginning to dissolve.
Add the eggs and whisk continuously over a gently heat until the egg is incorporated and the mixture thickens. About 10 minutes. Don’t let the mixture boil or the egg white will set in strings and you’ll have to strain it.
Lime curd isn’t lime green, its yellowish. If you want a more limey colour add a drop or two of green colouring.
Cool and store in a screw top jar in the fridge.
You can easily double the mixture if you have a surplus of limes.