Fill a small pot with about 1 inch of water and place on the stove over medium heat.
I a larger glass bowl,( you will want it big enough to set on top of the pot of water on the stove without the bowl touching the water ). Combine the lemon juice, lemon zest, sugar and eggs, whisk well.
Place 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 will take anywhere from 10 to 30 minutes. Once it has started to thicken, start adding in the butter.
Add the butter, one cube at a time, waiting until one cube is almost melted before adding the next. Once all the butter has been incorporated, transfer the curd to a jar or container and refrigerate until cool.