Start by zesting the lemons—all of the bright yellow outer layer but none of the bitter white pith. Then, juice the lemons and set the juice aside.

In a saucepan, combine the lemon juice, zest, sugar, eggs, and egg yolk. Whisk them all together until well blended.

Drop the pieces of butter into the mixture. Place the saucepan over medium-low heat, stirring continuously until the butter melts and the mixture begins to thicken (about 10 minutes).

You want your lemon curd to coat the back of a spoon. If you dip a spoon in and run your finger down the back, it should hold a line.

Once thickened, remove from heat and strain the curd through a fine sieve into a bowl to remove any lumps or bits of cooked egg.

Let it cool to room temperature before transferring it to a jar. Seal tightly and refrigerate.