In a heatproof bowl, whisk together the lemon juice, lemon zest, sugar, and eggs.
Place the bowl over a pot of simmering water (double boiler) and stir continuously until the mixture thickens.
Add the butter to the thickened lemon mixture, stirring until the butter is fully melted and incorporated.
Continue to cook and stir until the lemon curd reaches a consistency that coats the back of a spoon.
Remove the bowl from heat and let the lemon curd cool slightly.
Once cooled, transfer the lemon curd to jars or airtight containers and refrigerate.