In a medium bowl, combine lemon juice, sugar, eggs, and salt, whisking well.
Very slowly add butter in small amounts to egg mixture, whisking constantly so eggs don’t curdle.
Return mixture to saucepan. Cook over medium-low heat, stirring constantly until thickened, approximately 10 minutes. If mixture begins to scorch, lower heat. (Thickened mixture should coat back of the spoon and hold when finger is drawn through mixture.)
Pour mixture though a fine-mesh sieve into a bowl. Cover, and refri-gerate until needed, up to a day.
Recipe by TeaTime Magazine at https://teatimemagazine.com/3-lemon-tartlets-recipe/