Simple American buttercream frosting is usually a safe bet, but when you venture into European territory, you're asking for trouble. Swiss, French and Italian buttercream frostings rely on an emulsion of eggs, butter and sugar, and any number of things can go wrong. If you're buttercream has turned to butter, the problem is usually the temperature of the ingredients. To fix it, warm the frosting up or cool it down, depending on its appearance.
The most common reason buttercream turns to butter is simply because the butter is too soft or too hard. If the butter is too soft, you see pools of oil on top of the frosting and the frosting has a limp, soft texture like melted butter. If the butter is too hard, the frosting breaks down into grainy lumps that look like butter. The butter should be at room temperature -- neither cold nor warm.
Fixing Melted Buttercream
One of the simplest solutions for curing a melted batch of buttercream is to add a bit of cool -- not cold -- butter to the frosting. Allow 1/4 to 1/2 cup of butter to rest on the countertop for 30 to 60 minutes, or until it's just soft enough that your finger leaves a small indentation in it if you touch it. For a quicker fix, you can soften it in the microwave if you work carefully in 10- to 15-second intervals. Don't overdo it, though, or you'll soften it too much. Another simple fix is to add some shortening. Butter has a melting point between 88 and 98 degrees Fahrenheit, while shortening remains firm to 106 F. Adding shortening helps firm the softened butter and restore your frosting. Place the frosting in a stand mixer, or use a hand mixer, and beat it until it's light and fluffy.
Fixing Cold Buttercream
If the frosting has separated into grainy lumps, blow hot air from a hair dryer onto the bowl for a few seconds. The hot air melts the butter so that it can then be whipped into the frosting. Some stand mixers are powerful enough that you may be able to fix the frosting just by beating it longer. The frosting heats up slightly from the energy generated from the mixing blades.
An Ounce of Prevention
To prevent buttercream disasters, start with room-temperature ingredients. If you're making Swiss, Italian or French buttercream, use real eggs or egg whites, not meringue powder, and add a bit of cream of tartar to the eggs. Allow the meringue to cool to around 85 F, but no cooler, before adding the butter. Meringue that's either too hot or too cold can curdle the frosting. Sometimes the only thing you need is a bit more patience. When you first start adding butter to a cooked buttercream frosting, it may appear broken initially. Keep mixing and adding butter, and it will probably turn into light, fluffy frosting without any extra help.