In a medium, heavy-based saucepan, cook the mixture over low heat until it looks smooth. (The curdled appearance disappears as the butter in the mixture melts.) Increase the heat to medium and cook, stirring constantly, until the mixture thickens, about 15 min. It should leave a path on the back of a spoon and will read 170°F on a thermometer. Don’t let the mixture boil.

Remove the curd from the heat; stir in the lemon zest. Transfer the curd to a bowl. Press plastic wrap on the surface of the lemon curd to keep a skin from forming and chill the curd in the refrigerator. The curd will thicken further as it cools. Covered tightly, it will keep in the refrigerator for a week and in the freezer for 2 months.

Variations

For lime curd, substitute fresh lime juice and zest for lemon.