From America's Test Kitchen Season 9: French Country Cooking
A classic meringue cookie may have only two ingredients—egg whites and sugar—but it requires precise timing. Otherwise, you’ll end up with a meringue that’s as dense as Styrofoam or weepy, gritty, and cloyingly sweet. A great meringue cookie should emerge from the oven glossy and white, with a shatteringly crisp texture that dissolves instantly in your mouth. We chose a basic French meringue over a fussier Italian meringue. The French version, in which egg whites are whipped with sugar, is the simpler of the two; the Italian meringue, in which hot sugar syrup is poured into the whites, produces cookies that are dense and candy-like. The key to glossy, evenly textured meringue was adding the sugar at just the right time—when the whites have been whipped enough to gain some volume, but still have enough free water left in them for the sugar to dissolve completely. Surprisingly, we found that cream of tartar wasn’t necessary. Without it, the whites formed more slowly, giving a wider time frame in which to add the sugar. It was also important to form the cookies in a uniform shape, so we piped them from either a pastry bag or a zipper-lock bag with a corner cut off.
Makes 48 small cookies
Meringues may be a little soft immediately after being removed from the oven but will stiffen as they cool. To minimize stickiness on humid or rainy days, allow the meringues to cool in a turned-off oven for an additional hour (for a total of 2) without opening the door, then transfer them immediately to airtight containers and seal. Cooled cookies can be kept in an airtight container for up to 2 weeks.