Have ready a fine-mesh strainer suspended over a medium bowl containing the lemon zest.

In a medium heavy saucepan, whisk the egg yolks, sugar, and butter until well blended. Whisk in the lemon juice and salt. Cook over medium-low heat, stirring constantly with a silicone spatula and scraping down the sides of the pan as needed, until thickened and resembling hollandaise sauce, which thickly coats the spatula but is still liquid enough to pour. The mixture will change from translucent to opaque and begin to take on a yellow color on the spatula. Do not let it come to a boil or it will curdle. Whenever steam appears, remove the pan briefly from the heat, stirring constantly, to keep the mixture from boiling.

When the curd has thickened and will pool thickly when a little is dropped on its surface (an instant-read thermometer should read no higher than 196°F/91°C), pour it at once into the strainer and press it through into the bowl. Stir gently to mix in the zest and let the curd cool for 30 minutes. There will be ½ cup/4.6 ounces/132 grams.

Cover tightly and refrigerate, whisking occasionally to prevent the curd from setting, until cooled to room temperature, 20 to 30 minutes.