Lemon curd is a thick, soft and creamy, spreadable cream that has a wonderful lemon flavor. It was traditionally used as a spread for scones but nowadays you find it used as a filling for tarts and cakes, or even as part of a trifle. Lemon Curd is like a lemon filling or custard in that it does not use special ingredients; just eggs, sugar, lemon juice, lemon zest, and unsalted butter. And like a custard it is cooked on the stove, only instead of directly over the heat, I cook it over a saucepan of simmering water(double-boiler). I know that doing it this way takes a little longer, but I find it helps prevent the eggs from that dreaded curdling (little specks of cooked egg). Just make sure that the water in the bottom saucepan is 'simmering' which is defined as the point just short of a boil, that is, when bubbles start to appear. If I find the lemon curd is not thickening fast enough, all I do is increase the temperature of the simmering water. Once the lemon curd has become nice and thick (like hollandaise), remove it from the heat and strain to remove any lumps that may have formed. Then stir in the butter and lemon zest and your done. Cover immediately with plastic wrap to prevent a skin from forming and refrigerate. You will find that the lemon curd will continue to thicken as it cools. You can even make the lemon curd lighter in texture and flavor by folding in a little whipped cream once the curd has been thoroughly chilled. When choosing lemons look for ones that are fragrant with brightly colored oily yellow skins. The best ones are firm, plump, and heavy for their size. Don't buy lemons that have blemishes, soft spots, or are hard and wrinkled. Lemons consist of a yellow outer rind (skin) that can be of varying thickness and graininess, and can have either a bumpy or a smooth texture. This outer skin is where most of the lemon's wonderful tangy flavor is located. Before removing the outer rind (zest) make sure you wash the lemon thoroughly (soap and water is best). When removing the zest do not remove the white membrane (pith) that is underneath as it is very bitter tasting. Once you have removed the outer rind, inside the lemon are small vessels called 'pulp vesicles' which contain the pleasantly acidic lemon juice and seeds. Squeezing the lemon by hand or with a lemon squeezer or reamer releases this clear tart juice. Always use fresh lemons to make lemon curd as the bottled lemon juice is a poor substitute and won't give you that wonderful sharp and refreshing citrus taste.