Fill a small pot with about 1 inch of water and place on the stove over medium heat.

In a larger glass bowl, (you want one that will sit on top of the pot on the stove without touching the water), combine the lemon zest, lemon juice, sugar and eggs. Whisk well to combine. Place the bowl on the pot on the stove and start whisking.

Whisking the whole time, cook until the mixture starts to thicken and will coat the back of a spoon. This can take anywhere from 10 minutes to 30 minutes. When it starts to thicken, feel free to switch to a spatula, or you can use a whisk the whole time.

Once it has thickened enough to coat the back of a spoon, start adding in the butter, one small piece at a time, whisking until it is almost melted before adding another piece.

Once all of the butter has been incorporated, transfer the curd to a glass jar or lidded container and refrigerate until cool.