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 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 the water reaches a simmer, reduce the heat to low and place the bowl on top of the saucepan. (The bowl should be large enough to fit on top of the 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 the heat and stir in the butter, one 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.