Add the sugar, egg, and salt to a small saucepan (see notes on what type) and whisk together until smooth.

Then turn the stove on medium-low heat and immediately add the lime juice and zest and start whisking. 

Whisk continuously over medium to medium-low heat. The mixture will become frothy as you whisk. When the liquid becomes hot enough to cook the egg, the mixture will begin to thicken and no more liquid will separate from the foam.

It will take at least 5 minutes of whisking over medium heat for the mixture to thicken, but will vary depending on your cookware and your stove.  Don't be tempted to turn the heat up too high, just be patient. At around 5 minutes, the mixture should be thick enough to coat the back of a spoon. 

Then remove the pan from the heat. Add one tablespoon of butter at a time, whisking until each has fully melted into the sauce before adding the next. As the butter is melted into the sauce, it will become less frothy and more silky in texture, and look more glossy rather than foamy.

Once all of the butter has been whisked in, add a tiny drop of green gel color (if you want it to be pale green, indicating the lime flavor).  This is optional, you definitely don't need to do it. 

Transfer the curd to an air-tight container and refrigerate until completely cooled (2-3 hours at least).  It will thicken more once it's cool.