Add the egg-white powder, cornstarch and cream of tartar to the bowl of a stand mixer with the whisk attached, mix briefly to combine, and pour the water over it. Start mixer on medium-low and mix until the egg powder is incorporated and mixture is foamy; scraping down the sides with a spatula several times. Once the egg powder is pretty well mixed in, turn the mixer up to high and whip until you have soft peaks, a process that will only take a few minutes. Gradually add the granulated sugar and whip until very stiff. This beating-until-stiff step is probably the most important one in this recipe, because there’s no way to go back later and fix it. Beat in the powdered sugar.
Add the shortening, if using, bit by bit, then add the butter piece by piece while keeping the machine running. Some recipes have you switch over to the paddle attachment at this point but I see no reason to do this. The frosting will get a little curdled-looking if the butter was cold. That is perfectly fine. Just keep whipping and it will smooth out. If the butter was too soft and your frosting is collapsing, take the bowl off the stand and put it in the fridge for 20 minutes or so and try again. (But if you’ve done it the way I’ve told you to this won’t happen.) At some point a magical transformation should take place in which the mixture suddenly becomes stiff, with the whisk leaving a trail. But if not, and if the fat is in the fire and time is short, you can also add more powdered sugar if your frosting is still puddle-like, but you want to avoid that if you can, as you’ll lose the lovely texture of this type of frosting. Beat in the powdered raspberries, the raspberry or vanilla extracts, and the liqueur, if using.
*This amount of egg-white powder to water is more than you will find listed on the package; The brands of dried whites that I’ve used have the proportions as 1 T. powder to 3 T. water, which has given me overly-soft results, so I’d advise that you ignore their directions and follow mine.
**These 2 ingredients add stability to the meringue, the cream of tartar with acidity (which also cuts the sweetness) and the cornstarch with, well, starch. They are not absolutely necessary but I make it a practice to add them.
***This is another “insurance” product, not so much powdered sugar that you start to get the dreaded grittiness but just enough to do a little more stabilizing. (Can you tell that I’m paranoid about frosting stability?)
****I don’t so much use the shortening as stability insurance (although it does do that job) but as a way of toning down the butter flavor. The neutral-flavored shortening used as just one-fourth of the fat tones that down a little bit. But I don’t use Crisco! I never use Crisco, under any circumstances. I use something called “Spectrum Organic All-Vegetable Shortening,” which is a non-hydrogenated solid shortening made of something called “palm fruit oil.” (It’s kind of expensive, but you won’t use it for anything other than this recipe, you don’t use much at a time, and it keeps forever.) I don’t really know anything about the healthfulness of this product, but that’s not my point. It has a lovely, creamy-yet-solid texture and a soft white color, as well as a neutral flavor, so it not only tones down the butter flavor but also the butter color. You’ll never get a pure white frosting using butter (unless you do something weird with food coloring—let’s not go there, okay?), but you’ll get something at least a little whiter. And BTW, just in case you’re wondering, I wouldn’t advise using coconut oil as your shortening. It melts way too easily and can add a definite coconut flavor note that you probably don’t want.