Step 1

In a medium (2-quart) saucepan, whisk together the sugar, eggs, lemon zest, lemon juice, and salt. Set the pan over low heat. Cook, whisking constantly, until the mixture is thickened enough to coat a spoon (about 10 minutes). The lemon curd will thicken at about 170°F or just below a simmer. Off the heat, add the butter all at once and whisk until melted and evenly incorporated. Pass the lemon curd through a fine mesh strainer to remove the lemon zest, if desired. Cover and refrigerate until cool and thickened, at least 4 hours or preferably overnight.

Step 2

Make-Ahead/Freezer-Friendly Instructions: Lemon curd will keep in an air tight container in the refrigerator for about 2 weeks. To freeze for longer storage, transfer the lemon curd to a freezer-safe container, leaving about ½ inch of space at the top to accommodate expansion. Store in the freezer for up to 3 months. When ready to use, thaw the frozen lemon curd overnight in the refrigerator.