I was asked to make a cake covered in buttercream; which I had successfully done in the past. I used a recipe I had previously used for Italian Buttercream and it was a hit; the only change-up was that I added some veg shortening because the cake would be sitting in the sun and didn't want it to melt with all butter. The night prior to the delivery I had already dirty iced my cake, but because it was so late, I decided to finish icing it the following day (so I put the cake & icing in the fridge). I re-whipped the icing in the morning, and as I started to ice my cake, I noticed that the icing I was spreading was curdled-looking (and the one on the cake was perfect). I thought that maybe the icing was still cold and that the butter/veg shortening combo I used was solidified so I let it sit for a moment. I continued to re-ice and the BC was still curdled! To my horror, when I had finished icing the cake, it started to seep or should I say a watery liquid stated to come from the icing. Considering, I had already made this icing, I have no idea what I did wrong. Was it the butter/veg combo? Was it that it chilled in the fridge? So confused...and disappointed! Although the icing tasted fabulous, it looked like elephant skin on a cake. Has this happened to anyone before???

You have to re-whip IMBC that has been refrigerated.

From a CC post in 2009:

"Just keep whipping it. It'll come back together once all the ingredients warm up and reach the same temp. If you are in a hurry then you can heat the bowl with a torch or warm cloth. It will always seperate and look terrible when you first start re-beating it. "

Rae

It always separates if you try to do it when the ingredients are slightly cold. Best thing to do is take it out the night before then whip it in the morning using the beater blade not the egg whisking one.