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I don't think a baker can ever have too many recipes for any given dessert, and that's as true of buttercream as it is of chocolate chip cookies. That's because what's "best" can vary depending on what ingredients one has on hand, how much time is available for the project, and/or what dietary considerations may be required, not to mention the degree of chocolate intensity your current craving may demand.
After you've made a yolk-rich yellow cake, it's only natural to give those wayward egg whites a purpose with a batch of Swiss meringue buttercream spiked with chocolate. Likewise, a white cake based on egg whites may necessitate a chocolate twist on French buttercream instead.
But there are many other ways to make an eggless chocolate icing—some of which may require a bit of skill in the candy-making department, such as the old-school fudge frosting and new-school marshmallow buttercream from my cookbook, BraveTart: Iconic American Desserts. We even have an eggless chocolate frosting here on Serious Eats that's based on homemade sweetened condensed milk.
While all of these recipes can deliver big chocolate flavor, those techniques can be time-consuming, to say the least. Which is what brings me to yet another approach to chocolate frosting, this time using a fast and easy, thermometer-free technique that combines the intensity of ganache (a mixture of cream and chocolate) with the speed of an American buttercream (a mixture of butter and powdered sugar).
For those on the fence about American buttercream, which has something of a reputation for excessive sweetness and a slightly gritty consistency, rest assured that those qualities aren't inherent to the style. Rather, they reflect high-sugar/low-moisture formulas, as well as recipes based on conventional powdered sugar.
You can read a more in-depth walk-through of my solution to these issues with my own approach to American buttercream, but it largely hinges on using less sugar, more liquid, and an organic, tapioca-based powdered sugar.
That last bit is a subject worth exploring all on its own, so you may want to read up on the differences between organic and conventional powdered sugar as well.
Suffice it to say, my approach to a chocolate version of American buttercream takes advantage of all these techniques, with a not-insubstantial amount of high-fat Dutch cocoa powder added to the mix as well. But instead of a last-minute splash of cream to help dissolve the powdered sugar, I take the opportunity to incorporate an extra quarter pound of chocolate via a dark ganache.
The key here is to start with a good-quality chocolate bar rather than chips, one with a cocoa percentage somewhere around 70%. Any lower, and the frosting will be too sweet, while edging higher can cause the frosting to seize; for some of our favorite easy-to-find brands, see the Serious Eats guide to the best supermarket dark-chocolate bars for baking.
Making the ganache is a ridiculously simple affair: Finely chop the chocolate, then, off heat, whisk it into a pot of cream that's been brought to a bare simmer. When the mixture is silky-smooth, set the pot aside to cool.
Truth be told, this recipe will work with ganache in a wide range of temperatures, so long as the butter temperature is adjusted to compensate, but that kind of temperature-averaging can be tricky for beginners. So, for safety's sake, I recommend cooling the ganache to about 75°F (24°C).
This can be done passively, simply by setting the pot of ganache aside, or the process can be sped along by scraping the ganache onto a plate, where it can be spread into a thin layer for more rapid cooling.
In either case, with the ganache out of the way, sift the powdered sugar and cocoa over a bit of softened butter.
Again, a wide range of temperatures for the butter can technically work, particularly when you're using a warmer ganache. But for beginners especially, it's best to eliminate variables where you can, so I recommend using butter that's about 65°F (18°C).
You might be tempted to think that sifting is a superfluous step, and that any lumps of cocoa or powdered sugar will be beaten out with a stand mixer's paddle attachment, but take it from me: Not so. Organic powdered sugar is particularly prone to lumping, so please, do take the time to sift at this stage!
After sifting, I beat the powdered sugar, Dutch cocoa, and butter together until it's creamy, soft, and pale, scraping as needed with a flexible spatula along the way. The timing of this step will vary depending on the power of a given mixer, so, while you can expect it to take five or six minutes, the visual and textural cues are the most important here.
If you stopped at this stage, you'd have a very nice cocoa frosting. But the addition of ganache pushes it over the top, improving both its flavor and its consistency.
For a dense, dark frosting, stop whipping as soon as the ganache is incorporated. For one that's lighter in texture and color (if not flavor), keep whipping a few minutes more. This step is strictly a matter of your personal taste in appearance and consistency, so play around to find what you like best.
As with any buttercream, the temperature of the ingredients will have a profound impact on texture, and it can be a bit of a balancing act to get things just right. Don't hesitate to warm or cool the frosting, then re-whip until you get it where you want it.
Along those same lines, the final stage of mixing is a good time to give the buttercream a try so it can be seasoned to taste with additional salt or vanilla, as well as instant espresso powder in moderation. (The idea is to add a note of bitterness to round out the chocolate, rather than to make an overtly coffee-flavored frosting.)
In the end, you'll have a fast and easy chocolate buttercream, with a smooth consistency, that tastes rich and complex thanks to the dueling flavor profiles of Dutch cocoa and dark chocolate. It's perfect for everything from simple sheet cakes and cupcakes to layer cakes of any kind, making it a worthy addition to any baker's recipe box.