Melt the butter in a heavy medium saucepan over medium heat. Remove the pan from the heat.

Whisk cornstarch in lemon juice. Then whisk that, the sugar, zest, and salt into the melted butter.

Whisk in the yolks until smooth.

Return the pan to medium-low heat and cook, whisking constantly, until the mixture thickens, 5 to 6 minutes. Do not let it boil.

To check if the curd is thick enough, dip a wooden spoon into it and draw a your finger across the the back of the spoon; your finger should leave a path.

Immediately force the curd through a fine sieve into a bowl, using a rubber spatula. Let cool at room temperature, whisking occasionally. Refrigerate, covered, until ready to use.

MJF Note: I added the cornstarch to be sure it was thick enough.