Using a stand mixer, cream the butter until smooth and fluffy. Beat in the sugar and lemon zest and blend until smooth, scraping sides down half way through.

On low speed, add the egg yolks one at a time. Then add the lemon juice. Mix until combined.

Add about 1/2 inch of water to a pan and set a non-reactive metal bowl on top. You want a double boiler set up so that the steam from the water cooks the lemon curd; not direct heat.

Add the lemon mixture to the bowl and heat the water so that it boils, whisking the lemon mixture the entire time. Be sure to use an instant read thermometer. The curd should thicken around 170 degrees F, after about 10 minutes of cooking and whisking, and this will yield lemon curd that is on the thinner side. If you want your lemon curd to be a bit thicker and less runny, bring the temperature up to 180 degrees F before removing from the heat, still constantly whisking.

Remove the curd from the heat, transfer to a glass container, cover the surface of the curd with plastic wrap so that a skin doesn't form, and refrigerate until cool.

Store in an airtight container in the refrigerator for about a week or so. Alternatively, you can freeze lemon curd and thaw in the refrigerator before using.