This smooth, creamy mixture of lemon, eggs, sugar, and butter has a refreshingly bright, tart flavor.
In a 3-quart nonreactive saucepan, heat the lemon juice and butter over medium-high heat until just under a boil. In a medium heatproof bowl, whisk the eggs and egg yolk and then slowly whisk in the sugar until combined. Gradually whisk the hot lemon juice mixture into the sugar and eggs.
Return the mixture to the saucepan and set over medium heat. Cook, stirring constantly with a wooden spoon and scraping the bottom of the pan frequently, until the curd thickens and coats the spoon, 2 to 4 minutes. Draw your finger along the back of the spoon; when the curd is done, it should hold the trail.
Remove the curd from the heat and strain it through a fine sieve into a bowl. Whisk in the vanilla and salt. Cover with plastic wrap, pressing the plastic onto the surface of the curd, and chill for 1 to 2 hours before using.
Make Ahead Tips
The curd can be made up to 5 days ahead; refrigerate in an airtight container.
Add 3/4 tsp. finely chopped fresh rosemary to the saucepan with the lemon juice and butter and bring the mixture to just under a boil. Remove from the heat and let sit for 1 hour. Bring the mixture to just under a boil again, and continue as directed.
Photo: Colin Clark