My mother always made 7-minute icing, soft and fluffy and reminiscent of a big wedding dress. It is unapologetically sweet and miraculously light and smooth. This icing is technically an Italian meringue and it gets shinier and denser the longer and you whip it, so take your time. The cream of tartar helps control the crystallization of the sugar. The temperature will mount slowly and then rapidly as it approaches the end, so keep an eye on it. This recipe makes enough for two 6-inch cakes. Leftovers do not hold in the refrigerator as they lose volume, so serve it that day if possible. Unfortunately we could not scale this recipe for less than 4 cups, so you will have extra if you make just one 6-inch cake.
Put the egg whites and vanilla in a stand mixer fitted with the whisk attachment. When the sugar reaches 240 degrees F, immediately pour it into the measuring cup to prevent it from getting hotter. With the mixer on medium speed, slowly pour the sugar syrup into the egg whites, aiming for the side of the bowl rather than the whisk. When all the syrup is added, turn the mixer to medium-high and whisk until the icing becomes thick and holds a firm peak. Continue to whisk until the icing is just slightly warm and very thick, about 10 minutes total. Do not continue to beat, or the icing will become too thick to spread and pipe.