Most bakers are familiar with meringue-based buttercreams, like Swiss and Italian, but their French cousin isn't nearly as well known. In part, that's because Swiss and Italian buttercreams are cheap and practical, turning leftover egg whites into something useful and delicious—frosting for cake.
French buttercream, on the other hand, is made from whipped egg yolks, and egg yolks have many other, more popular uses: vanilla custard, lemon curd, chocolate pudding, or even yellow cake. It's not often that home bakers have a dozen egg yolks to spare, but it's been known to happen—in the aftermath of making angel food cake or macarons, for example. But French buttercream is also insanely delicious, with a rich custard flavor like the best vanilla ice cream, so it's worth having a recipe up your sleeve, or at least on your radar.
Classically, French buttercream is made by pouring a hot syrup over whipped egg yolks, but I make mine over a water bath instead. It's a method similar to my technique for Swiss buttercream, one that eliminates the mess (and risks) associated with pouring a boiling sugar syrup into a running mixer. It's also a method that ensures those egg yolks are fully cooked, something that can't be said for traditional French buttercream.
The first step is to combine the egg yolks with sugar, salt, brandy, and vanilla. The brandy could just as easily be bourbon, rum, or even water—anything liquid to help loosen the thick yolks. I like brandy best, because it's flavorful and aromatic but subtle, adding depth of flavor to the buttercream without getting in the way.
The mixture is set over a steaming water bath and stirred constantly with a flexible spatula until it hits 155°F (68°C). This temperature ensures that the sweetened egg yolks have fully cooked, as sugar will alter their normal cooking temperature.
When the mixture comes to temperature, it will have transformed from a thick and grainy paste into something almost like a syrup, smooth and loose but thick.
Immediately transfer it to a stand mixer, and whip on the highest speed until the egg yolk mixture is thick, foamy, and just beginning to ball up on a whisk. This takes about eight minutes on my KitchenAid, but the exact timing can vary considerably with the wattage of your mixer, so let the visual cues be your guide.
Once the egg yolks are fluffy, begin adding cool butter a few chunks at a time. The egg mixture is typically around 90°F at this time, so I like to use 65°F (18°C) butter, but there's a lot of wiggle room when it comes to temperature—so long as the two average out to something in the 70s Fahrenheit.
The mixture may look a little strange at first, but as the butter is incorporated, it will eventually become glossy, smooth, and thick enough to hold stiff peaks.
Buttercream is the least mysterious recipe to troubleshoot. If it's soupy or thin, it's too hot. If it's curdled or greasy, it's too cold. Either problem is easily solved by warming or cooling the buttercream, so there's never any reason to panic or despair. If your buttercream isn't the perfect consistency for spreading over a cake, just follow my troubleshooting guide for Swiss buttercream. French buttercream may start with different ingredients, but its backbone is still butter, so it can be fixed in exactly the same way.
Properly made, French buttercream should weigh about six and a quarter ounces per cup. If yours turns out even lighter, that's fantastic, but if it's significantly more dense, that's a sure sign the buttercream was too cold to be properly whipped. Again, this is easily remedied by warming and re-whipping the buttercream, per the troubleshooting guide.
Once it's been whipped to the perfect consistency, French buttercream is light and silky, with a dreamy, melt-in-your-mouth texture. It's wonderful as is, but also easily customized with ingredients like melted chocolate, peanut butter, ground nuts, or spices added to taste—with cinnamon and nutmeg to play off the brandy, it's also the perfect eggnog-flavored frosting. Pair it with anything from classic vanilla butter cake to chocolate cherry cake, or even a carrot cake for those who aren't keen on cream cheese frosting.