Add enough water to a medium saucepan to measure 1-inch deep and bring to a simmer over medium-high heat. Combine the egg yolks, sugar, lemon juice and zest in a medium metal bowl and whisk until smooth. Place the bowl on top of the saucepan (make sure the bottom of the bowl is not touching the water) and whisk until the curd is thickened, light yellow and coats the back of a spoon, about 8 minutes. Remove the curd from the heat and use a spatula to stir in the butter, one piece at a time, allowing each addition to melt before adding the next. Transfer to a clean container and press a sheet of plastic wrap directly against the surface of the curd. Chill thoroughly, at least 3 hours.
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