How to Make It
In a heatproof bowl, whisk together the eggs, sugar, lemon zest and juice, and salt. Add the butter. Set the bowl over (not in) a saucepan of simmering water and cook, whisking constantly, until the mixture has thickened to the consistency of mayonnaise, 12 to 15 minutes.
Pour the mixture through a fine-mesh sieve into a medium bowl. Place a piece of parchment or wax paper directly on the surface of the lemon curd and refrigerate until completely cool, at least 2 hours and up to 2 days.