Lemon Curd Macaron Filling

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.