Combine butter and sugar in a medium bowl or the bowl of a stand mixer and mix for two minutes.
Slowly add the beaten eggs / egg yolks, and beat for 1 minute more.
Mix in the lime juice. (It may look slightly curdled, but it will smooth out when cooked).
Transfer the curd to a heavy bottomed pan (non-stick works best) and heat, stirring constantly, until the sauce thickens – about 8 minutes. (It should be the consistency of pudding). Do not boil!
Remove from heat and stir in the lime zest, if desired.
Cool the lime curd at room temperature for 10 minutes, then transfer to a plastic or glass containers (canning jars work great) and cover tightly with a lid or saran wrap to prevent a skin from forming on top of the curd.
Will keep in the refrigerator for up to two weeks.