Swiss Meringue Buttercream (Base)
Back when I was making cakes for weddings and birthdays, I would often get asked about my frosting - mostly, "how sweet is it?" I think it's because a lot of people expect frosting to be crack-your-teeth sweet, and they're usually right. There are several different types of buttercream, but most shops choose to use American, which is composed of powdered sugar and butter or shortening. You'll see it on cakes, cupcakes, between cookies, and even macarons these days. American buttercream can definitely have its place depending on the application, and it's by far the easiest to make. But I always end up scraping the majority of it off because it's way too sweet for my taste.
Enter the meringue buttercreams. They're silky, rich, and perfectly sweet. There's nothing sickeningly overpowering about them, and they're the creamiest blank slate for any flavoring you'd like to add. And even though a meringue buttercream is soft and smooth, it can hold up beautifully for piping, too (especially Swiss). It's the only style of frosting I use, apart from the occasional ganache.
There are three kinds of meringue buttercream: Italian, French, and Swiss. The basic process for each is that you first make a cooked meringue of eggs and sugar, then add your softened butter and flavoring. Simple enough. The difference between them is that Italian meringue involves cooking the sugar to firm ball stage (245°F), then adding it to foamy egg whites with even more sugar. On the other hand, Swiss meringue involves cooking both the sugar and egg whites in a bain-marie (over simmering water), then whipping them to stiff peaks before adding the butter. Basically, it tastes like marshmallow fluff with butter added to it. How could it possibly be bad? Then there's French meringue, which is another beast all together. Egg yolks are whipped with cooked sugar, and even more butter is used than in the other variations. It seems fitting that the richest of the three is named after the French. Coincidence? Mais non.
I choose to make Swiss meringue over the others because it's a little more neutral, and it's the most stable since the egg whites and sugar are combined at the very begining. And most importantly, it ensures proper food safety by cooking the egg whites to 160°F (as opposed to 140°F or less, according to some recipes). I can rest assured knowing that if I choose to leave a decorated cake out on the counter overnight, I won't have to worry about salmonella beasties ruining it.
Dealing with hot sugar may seem a little frightening if you've never done it before. But it's really not that daunting. And I can promise you that once you start making meringue buttercreams, you'll never want to go back to using powdered sugar. Ever.
Swiss Meringue Buttercream (Plain)
Yields approximately 4 cups
Enough to ice a tall 6-inch cake or a short 8-inch cake (without additional piped decorations)
4 large egg whites (about 145 g)
200 g (1 C) granulated sugar
1/4 tsp cream of tartar
395 g (3 1/2 sticks) unsalted butter, room temperature (see Note)
Additional flavoring of your choice (extracts, fruit purées, chocolate spices, etc.)
Fill a small pot a quarter of the way with water, then set it over high heat. Allow it to come to a boil, then reduce to a simmer.
Meanwhile, separate your egg whites into the clean, dry bowl of a stand mixer. Be sure not to get any of the yolks mixed in. If you want to be extra careful, separate each egg into small bowls first, then pour the whites into the mixing bowl. That way, if a yolk bleeds into one of the whites, the rest of them won't be ruined. Then, whisk in your sugar and cream of tartar until combined.
Set the mixing bowl over the pot of simmering water making sure that the water does not touch the bottom of the bowl. You may want to wear oven mitts for this next part. Using a clip on candy thermometer or, better yet, a digital instant read thermometer, keep checking the mixture's temperature while constantly whisking until it reads 160°F. It'll only take a few minutes, and it should be thick and creamy. Immediately remove the bowl from the heat, wipe the condensation off of the bottom, and attach it to your stand mixer. Clip on the whisk attachment and beat the mixture over medium-high speed for a few minutes, until stiff, glossy peaks are reached (see the fourth photo above). Once at stiff peaks, turn the mixer down to low and keep stirring the meringue until the bottom of the bowl has cooled to room temperature - just feel it with your hand. This will take around 10 minutes or so. The meringue must be at room temperature before adding the butter; otherwise the butter will melt and your frosting won't be salvageable.
Next, remove the whisk attachment (scraping off as much of the excess meringue as possible), and replace it with the paddle attachment. With the mixer on low speed, start adding your softened butter a few tablespoons at a time. Once you've added all of the butter, turn the mixer off for a moment, remove the bowl, and scrape down the sides with a spatula in order to incorporate all of the meringue. Reattach the bowl, and beat the mixture on medium-high speed until it becomes thick and fluffy. It may look as if the buttercream is thinning out or curdling at some point, but just keep mixing it; it'll stiffen up. Once stiff, stop mixing so as not to beat in too much air. At this point, you can add any flavoring you'd like - a few teaspoons of extract, a few tablespoons of fruit puree, melted chocolate, zest, ground spices, fresh herbs - the sky's the limit!
The buttercream can be left out overnight in a sealed container if you plan on decorating a cake the next morning. It can also be left out on a cake overnight as long as it's in a cool room. But if it's going to be out in the heat or saved for several days, keep it in the fridge. Just let it come to room temperature before trying to use or eat it. Enjoy!
You may see a lot of recipes that call for less butter. I don't know why that is, but I've always used this ratio and never had problems. It holds up well over several days, doesn't melt readily, and isn't too rich.