Melt the butter in a heavy medium saucepan over medium heat. Remove the pan from the heat and whisk in the sugar, lemon juice, zest, and salt. 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. 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. Don't let the mixture boil. 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.