If this happens to you, try following these steps:
1. Increase your mixer speed for 1-3 minutes.
2. Play with the temperature. All the ingredients should ideally be at room temperature, so:
• If the bowl of the mixer feels cold to the touch, put it over a hot water bath or wrap a hot towel around the base.
• If the bowl of the mixer feels warm to the touch, put it over an ice bath or add cold butter (instead of room temperature butter)
3. Add melted chocolate. Chocolate is an emulsifying agent and can help bond the fat and liquids together. Melt the chocolate and add it while the mixer is running.
4. As a very last resort (and if you have no other ingredients left to make another batch!), strain the butter cream to separate the liquids and the solids. Put the liquids back into mixer and increase the speed to medium. Slowly add the strained fats back into the liquid to create a new emulsion.
In our experience, one of the first three tricks will work. We haven't yet had to resort to #4 - fingers crossed!
Anyone else have a special trick for making a buttercream come together?
Related: How to Frost a Layer Cake