Raspberry Squares This recipe combines a few ordinary pantry ingredients for out of the ordinary results.
Raspberry PreservesAre some brands of raspberry preserves better than others for baking or spreading on toast?
Coffee MakersMaking coffee is a daily ritual in most households—find out which coffee makers produce the best coffee.
Bar CookiesEasy and comforting bar cookies are staples in many homes. We’ve discovered the secrets to chewy, fudgy brownies and crispy, tasty raspberry squares.
Chocolate Flavor DiffusionLearn some important lessons about baking with chocolate.

Find out more
*	About ATK
*	About COOK'S


Lemon Curd
Cooked over low heat and stirred continuously till thickened, lemon curd owes it bright flavor not only to lemon juice but to the oils released by finely grated peel. Butter further refines the flavor and texture. The result is a spoonable custard that can be spread on scones or used as a base for desserts. When baked, its color deepens and the curd sets up, remaining supple and creamy yet firm enough to be sliced. 
Several variables in the process of making lemon curd warranted exploration: the ratio of sugar to lemon juice, the proportion of eggs, the cooking method, the cooking temperature, and the point at which to add the butter.
The most straightforward variable was the sugar to lemon ratio. We wanted just enough sugar to offset the acid. That turned out to be roughly 2 parts sugar to 1 part lemon juice, plus 4 full tablespoons of finely grated lemon zest.
The eggs required a little more experimentation. A curd made with whole eggs alone had a light texture in the spoon and a gorgeous sheen, but it had a muted color and a texture most tasters described as "mayonnaise-like" when baked. Whole eggs plus whites produced a curd that was too firm, while an equal ratio of whole eggs to yolks was almost cloyingly rich. In the end, most tasters preferred a curd made principally with yolks and just a couple of whole eggs for structure. This curd was creamy and dense with a vibrant color, and it did not become gelatinous when baked, as did those curds made entirely with whole eggs. 
While some recipes recommend cooking the curd over the indirect heat of a double boiler, we found that direct heat was fine as long as we kept it moderately low and checked the temperature with a thermometer—at 170 degrees the curd reached the right consistency, something like that of a dessert sauce. 
Adding cold butter chunks to the still-liquid curd proved superior to whisking in the butter after stovetop cooking. Though the latter curd looked glossy and beautiful before it was baked, the butter aerated the filling, causing it to rise and overrun the shell’s borders while in the oven. 
For good measure: Three tablespoons of cold, raw cream stirred into the curd just before baking proved a winning touch. It cooled the just-cooked curd and lightened its texture to a celestial creaminess. If you don’t get around to baking a crust, just buy some fresh berries and grab a spoon.

Click here for a print-friendly verision
Read the recent recipes developed in America's Test Kitchen. Subscribe to Cook's Illustrated Magazine
To read all of the recipes from our second season, Order The America's Test Kitchen Cookbook