Place a fine mesh sieve over a bowl. Set aside.

In the top pan of a double boiler or a metal bowl, whisk the lemon juice with the sugar and eggs.

Place the bowl over a pan of simmering water (the bottom of the bowl should not touch the water) and cook for 8 to 10 minutes, stirring constantly. When the mixture has thickened to the consistency of yogurt, pour it through the sieve.

Add the lemon zest to the curd and add the butter a few pieces at a time, stirring after each addition.

Pour it into a bowl and place a sheet of plastic wrap directly on the surface to prevent a skin from forming.

Transfer the cooled lemon curd to a jar and refrigerate for up to 2 week. Or freeze the lemon curd in small amounts for up to 1 year.