Zest and juice the Key limes until you have a half-cup of juice.
In a heavy-bottomed medium saucepan, whisk together the egg yolks and sugar until well-combined.
Add lime juice to the pan and stir with a wooden spoon.
Gently heat the mixture over medium to medium-low heat stirring constantly until it begins to thicken and coat the back of a spoon, about 10 to 15 minutes. Do not let curd boil!
When curd has thickened, remove from heat and stir in the butter until melted.
Strain the curd through a fine-mesh strainer into a bowl to remove any pieces of cooked egg.
Mix the lime zest into the strained curd.
Pour curd into glass jars or plastic storage containers and refrigerate.
Notes
Makes a scant pint of lime curd
Recipe by West of the Loop at https://www.westoftheloop.com/2014/01/18/key-lime-curd-teatime-dessert-beyond/