Buttercream can split if the ingredients aren’t at the right temperature or if too much liquid was added at once. This is why I recommend adding ingredients in steps. To fix broken buttercream, try warming the mixture slightly by placing the bowl over a pot of simmering water, then beat it again until it comes together. If the buttercream is too warm, cool it in the fridge for a few minutes, then re-whip to restore its smooth texture.