In a heavy saucepan, beat the yolks and sugar until well blended. Stir in the remaining ingredients except for the lemon zest.

Cook over medium-low heat, stirring constantly until thickened and resembling a thin hollandaise sauce, which thickly coats the back of a wooden spoon but is still liquid enough to pour.

The mixture will change and become opaque and begin to take on a yellow color. It must not be allowed to boil or it will curdle.

You may have to remove the pan when you see steam for a moment and then replace it. When the curd has thickened, pour immediately through a strainer and add the zest. Discard the remaining residue. Pour into an airtight container and let cool. It will continue to thicken.