Separated buttercream might make you wonder why you didn't just order a cake from the local bakery, but once you understand why buttercream breaks, you can fix even the most hopelessly separated frosting. Sometimes you just need to be patient and keep beating -- buttercream always looks lumpy and separated just before it emulsifies.
Why Buttercream Breaks
Buttercream is an emulsion, or a stable mixture of water, fat and an emulsifier. When those ingredients don't mix together completely, what should be a silky smooth frosting turns into a soupy, lumpy mess.
Buttercream emulsions break for three main reasons: Butter is too cold Frosting is too warm * It doesn't have enough emulsifiers
Once you've determined which of these three is responsible for your broken buttercream, fixing it is relatively simple: Reverse the condition that broke it in the first place.
Cool Down the Buttercream
Overheating is a common culprit in broken buttercream. If the butter is too soft, or the mixture has been mixed for too long, or the hot sugar syrup is added too quickly, the whole bowl of frosting will become too hot to emulsify.
The bowl of frosting should be at room temperature or slightly below. If it's warm to the touch, put the bowl in an ice bath, or wrap a flexible ice pack around it for a few minutes until it cools down.
Warm Up the Buttercream
Buttercream freezes beautifully, with a warning: let it come all the way to room temperature before rebeating. If you beat the buttercream while it's still slightly chilled, the hard fats in the butter separates from the liquids and the emulsion breaks.
Use a hair dryer to gently warm the outside of the bowl until the edges of the buttercream begin to melt, then rebeat. If you don't have a hair dryer handy, microwave a small portion of the frosting until it is liquified. Pour the liquid frosting back into the bowl and beat, allowing the hot frosting to raise the overall temperature of the buttercream.
Add an Emulsifier
On the rare occasion that heat isn't the problem, you may simply need to add an emulsifier. Chocolate contains lecithin, which is a common emulsifier. Melt semisweet chocolate in your microwave and slowly beat it into the buttercream. You can incorporate up to 50 percent as much chocolate as butter. For example, if your buttercream recipe calls for two sticks, or 8 ounces, of butter, you can add as much as 4 ounces of melted chocolate.
If you're making a white icing, sprinkle a pinch of xanthan gum into the bowl and re-beat. Add a little at a time until the buttercream re-emulsifies. Xanthan gum is a natural emulsifier derived from plant matter and is available in the gluten-free aisle of most grocery stores.