Add enough water to a medium saucepan to come about 1 inch up the side. Bring to a simmer over medium-high heat.
Meanwhile, combine egg yolks and sugar in a medium-sized metal bowl and whisk until smooth, about 1 minute.
Measure lemon juice and, if needed, add enough cold water to reach 1/3 cup. Add juice and zest to egg mixture and whisk smooth.
Once water reaches a simmer, reduce heat to low and place bowl on top of saucepan. (Bowl should be large enough to fit on top of saucepan without touching the water.) Whisk until thickened and the mixture is light yellow and coats the back of a spoon, about 8 minutes.
Remove promptly from heat and stir in butter a piece at a time, allowing each addition to melt before adding the next. Remove to a clean container and cover by laying a layer of plastic wrap directly on the surface of the curd. Refrigerate for up to 2 weeks.