Place the egg yolks, whole eggs, and granulated sugar in a medium saucepan (off the heat). Using a whisk, whisk the mixture for 1 minute. Add the lemon/lime juice and the zest and continue to whisk for an additional minute until combined and sugar dissolves.

Place the saucepan on the low heat setting, switch to a wooden spoon and continuously stir the mixture as it cooks for 10-15 minutes. You can tell the lemon curd is done when it is thick enough to coat the back of the wooden spoon without dripping. If after 8-9 minutes the curd has not thickened much, you can turn up the heat to the medium low setting. Mine took exactly 13 minutes. You'll notice the curd is lightly shiny. When done right, there is no need to run the curd through a fine mesh strainer.

Remove the curd from the flame and stir in the cold cubed butter until the curd is shiny and smooth.