Prepare an ice water bath; set aside. Whisk together the egg yolks and sugar in a medium heatproof bowl. Add the lemon juice, butter, and lemon zest. Place over a pan of simmering water and cook, whisking occasionally, until thick, about 15 minutes. Transfer bowl to ice water bath and let stand, stirring occasionally, until cool. Place a piece of plastic wrap directly on the surface of the curd. Transfer to refrigerator until completely cold, at least 2 hours and up to overnight.