Pair this with store-bought madeleine cookies.
Whisk yolks, sugar, lemon zest, and lemon juice in a heatproof bowl set over a pan of simmering water. Continue whisking over medium-high heat until thick enough to coat the back of a spoon and an instant-read thermometer registers 160 degrees, 8 to 10 minutes.
Remove from heat. Add salt and the butter, one piece at a time, mixing until smooth. Strain through a fine sieve into a medium bowl. Place plastic wrap directly on surface of curd to prevent a skin from forming. Refrigerate at least 1 hour or up to 1 day.