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 size metal bowl and whisk until smooth, about 1 minute. Measure citrus 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, approximately 8 minutes, or until mixture is light yellow and coats the back of a spoon. 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.
Recipe courtesy of Alton Brown