In a saucepan over medium heat, bring 2 to 3 inches of water to a simmer.
In a heatproof medium bowl, whisk together the egg yolks and sugar until thoroughly combined. Whisk in the lemon juice and zest and set the bowl on the saucepan, making sure the bottom does not touch the simmering water. Cook, whisking occasionally, until the mixture is thick enough to coat the back of a spoon and the temperature registers 180°F on a digital thermometer, 20 to 30 minutes.
Strain the curd through a fine-mesh sieve set over a clean medium bowl. Add the butter a few pieces at a time, stirring with a rubber spatula. When all the butter has been incorporated, blend with an immersion blender until the curd is completely smooth, about 30 seconds.
Strain the curd once again, this time into a storage container. Press plastic wrap onto the surface and refrigerate until chilled and set, 3 to 4 hours.
Note: This recipe can easily be halved or quartered. Refer to the Modern Art Desserts recipe that you're making for the quantity of lemon curd that you'll need.Do ahead: Stored in an airtight container, the lemon curd can be held for up to 1 week in the refrigerator or for 4 months in the freezer.