In a large heat proof bowl, combine the lemon zest & juice, sugar, eggs and butter and place over (not touching) gently simmering water in a saucepan. Whisk steadily until the sugar dissolves and the butter melts, then continue to whisk until the curd coats the back of a spoon, takes about 3 minutes. Do not let the curd boil.

Remove from the heat, and using a rubber spatula, push the curd through a medium mesh sieve into a clean, dry bowl.

Cover with plastic wrap, pressing it directly onto the surface of the curd to prevent a skin from forming. Poke a few holes in the plastic with the tip of a knife to allow the steam to escape.

Refrigerate until well chilled, about 3 hours or up to 5 days.