Place a fine-mesh sieve over a heatproof medium bowl.

In the top of a double boiler, whisk together sugar, lemon zest and juice, egg yolks, and salt until well combined. Cook over a saucepan of simmering water, stirring constantly, until thickened and can coat the back of a spoon and an instant-read thermometer registers 170°F (77°C) to 180°F (82°C), 10 to 15 minutes.

Press curd through prepared sieve into bowl, discarding solids. Add butter, 1 to 2 cubes at a time, stirring until melted after each addition. Cover with a piece of plastic wrap, pressing wrap directly onto surface of curd to prevent a skin from forming. Refrigerate until chilled and set, at least 2 hours.