This recipe for Lemon Curd filling from Martha Stewart goes great with Lemon Meringue Cupcakes. I advise you make it the day before and save the whites for the frosting :)
8 large egg yolks
Finely grated zest of 2 lemons
120 ml plus 2 tablespoons freshly squeezed lemon juice (about 3 lemons)
225g caster sugar
1/8 teaspoon salt
155g unsalted butter, cold and cut into pieces
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 to coat the back of the spoon, about 8 to 10minutes, and registers 160 degrees (70c) 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.