This great recipe for lemon curd, from "Martha Stewart's Baking Handbook," can be used when making Lemon Meringue Cupcakes.
Combine yolks, lemon zest, lemon juice, and sugar in a heavy-bottom saucepan; whisk to combine. Cook over medium-high heat, stirring constantly with a wooden spoon (be sure to scrape the sides of the pan), until the mixture is thick enough to coat the back of the spoon, 8 to 10 minutes, and registers 160 degrees on an instant-read thermometer.
Remove saucepan from heat. Add salt and butter, one piece at a time, stirring until smooth. Strain through a fine sieve into a medium bowl. Cover with plastic wrap, pressing it directly onto the surface of the curd to prevent a skin from forming. Refrigerate until chilled and set, at least 1 hour or up to 1 day.