In a bowl, combine the granulated sugar, lemon juice, zest and egg yolks.
Place over a pot of simmering water and, whisking occasionally, cook until thickened considerably (about 10 minutes).
Remove from heat and allow to cool for about 10 minutes.
Cube the butter into 8 pieces.
Stir in the butter until completely incorporated.
Pour into a bowl or jar, and press a piece of plastic wrap onto the surface of the curd to keep a skin from forming.
Refrigerate before use and up to 7 days.
Notes
Don't use bottled lemon juice: The best lemon curd is made with freshly squeezed lemon. I promise it's worth the effort.
Estimating lemons: Depending on their size and how pithy your lemons are, you'll need 1-2 lemons to make 1/4 cup of juice.
Extra thick lemon curd: If you want an extra thick curd, substitute one egg yolk for a whole egg.
Use a fine mesh sieve for ultra smooth lemon curd: Because the zest is so fine, I usually skip this step. But if you want the smoothest possible lemon curd, run it through a mesh sieve before adding the butter.
Storage: Lemon curd can be refrigerated for up to seven days and frozen for up to three months.