In a medium bowl set over a saucepan of simmering water, melt the butter with the lemon juice and sugar, stirring occasionally, until the sugar is dissolved. Whisk in the egg yolks, 1 at a time, and cook over moderately high heat, whisking constantly, until the curd is very hot to the touch and the consistency of loose sour cream, about 15 minutes. Strain the curd into a bowl and press a piece of plastic wrap directly onto the surface to prevent a skin from forming. Refrigerate before serving.
The curd can be stored in the refrigerator for up to 1 week.