Whisk together juice, zest, sugar, and egg yolks in a heavy saucepan. Whisk in butter and cook over medium-low heat, whisking frequently, until curd is thick enough to hold marks of whisk and bubbles begin to appear on the surface, about 5 minutes.
Transfer lemon curd to a bowl and chill, pressing plastic wrap over the surface of the curd, at least 1 hour. Store in a jar in the refrigerator up to a week.
