Ethereal buttercream frosting

This is my new favorite frosting, inspired by some cupcakes from an unknown Cary bakery.  There were piled high with frosting, but it was very airy, not too sweet, and not too rich.  Even though there was a lot of it, it was delicious and not at all overwhelming.

This recipe was derived from one by King Arthur Flour, for 7 minute frosting, a.k.a. Italian meringue.  I increased the sugar a little, added cornstarch in hopes of stabilizing the meringue, and beat in some butter.  The recipe seems fairly robust at this point, and much less fussy than the Swiss meringue buttercreams I tried.  Also, with this proportion of sugar to egg white (and the whites starting at room temperature), the whole mix gets up to 155 F (in my kitchen at least), which should be enough to fully cook the egg and alleviate any food safety concerns.  However, if you're cooking for the very young, the very old, pregnant women, or immune-compromised individuals, you might consider pasturized eggs or whites for extra safety.

This frosting works best cold, at refrigerator temperatures.  Fully chilled, it's firm but still quite spreadable / pipeable.  I've kept it that way for a week with no problem.  It probably shouldn't be left at room temperature for more than a few hours, not least because at warmer temperatures the frosting starts to get a bit runny.  (The shortening should help with that somewhat.)  It also tastes a bit too sweet and marshmallowy for my liking when warm;  the amount of sugar is calibrated for serving it cold.

To start, cut the butter into tablespoons and separate the egg whites;  allow both to come to room temperature.  In the bowl of a stand mixer, place

1. 2 egg whites (about 2.5 oz)
2. 1/4 t cream of tartar

Whisk until foamy and increased in volume.  The whites should mound in the bowl, but ideally are not yet holding a soft peak.

In a medium saucepan, while whites are whipping, whisk together and bring to a boil

1. 1 T cornstarch
2. 9 oz sugar (1 1/4 c)
3. 2 oz water (1/4 c)
4. 1/8 t salt

Cover saucepan briefly to wash any sugar crystals from sides.  Try to time it so that syrup boils just as whites reach stiff peaks.  If needed, rest syrup off heat while whites finish;  underbeaten whites will keep the frosting from reaching full volume.  When sugar is completely dissolved and whites are fully whipped, slowly pour syrup down the side of the mixer bowl with mixer running on medium speed.  (Be careful not to splatter hot syrup on yourself!)  Increase speed to high and whip until meringue is thick, fluffy, and shiny.  Reduce speed to medium and continue whisking until cooled, about 10 minutes.  Frosting can be used as-is at this point, or ...

One tablespoon at a time, beat in

1. 1 oz vegetable shortening, such as Crisco (2 T)
2. 3 oz butter (6 T)
3. 1 t vanilla extract

The frosting will be bright white at this point, but it takes gel food color very nicely.  The frosting will deflate somewhat as the fats are added, and may become quite runny, depending on how warm the ingredients are.  (Butter may be substituted for the shortening, but this will exacerbate the runniness.)  No worries, the frosting will firm up when chilled in the refrigerator.  After chilling, use for frosting cakes, cupcakes, or even Oreo cookies!