METHOD:

1) Place sugar in a bowl, and rub together with the grated lemon.
2) Place lemon juice, eggs, yolks and sugar mixture together in a glass or "nonreactive" bowl, and place over a simmering waterbath.
3) With a plastic whip, whisk constantly until very thick. This takes about about 15 minutes and the mixture will reach 185F.
4) Strain into a clean bowl. Stir slowly and allow the mixture to cool to 110F. At this point, slowly whisk in the pieces of softened butter.
5) Refrigerate until needed, or pour into pre-baked tart shells and refrigerate until set - then glaze.

Variation:
Lightened w/ 16 oz. of whipped cream.