Put lemon juice, zest, sugar and butter into a heavy saucepan and place over heat-diffuser on medium heat. (You can also use a double boiler, but a heat-diffuser works well.) Stir frequently to mix ingredients and to prevent sugar from possibly burning.

In a small bowl mix eggs and egg yolks.

When the butter has melted and the sugar has dissolved, remove saucepan from stove and quickly whisk in eggs and egg yolks. Return to stove, whisking continually.

Cook mixture until mixture thickens enough to coat the back of a spoon. Remove from heat immediately and press through a sieve into a bowl.

Cover with clear plastic or waxed paper and allow to rest for 20 minutes. Add vanilla to taste.

Because this is a custard, occasionally mistakes happen and the eggs cook too quickly or the mixture curdles. If this happens, strain through a sieve a second time.

Depending upon the variety of citrus used, you may need a little more lemon or a little less sugar.