I was reading Brave Tart’s macaron recipes and technique and came across this tip (from the comments) on working with frozen buttercream—haven’t tried it, but she works with this stuff, professionally, on a daily basis, so I thought it was worth putting out there in case anyone wanted to give it a shot!
Hi Tejeanne! Yes, it is best to re-whip it before spreading. The best way to do this is to divide the frozen buttercream into thirds. Melt 2/3 of the buttercream completely (microwave or water bath), and then put it on the mixer to whip on low speed with the remaining 1/3 that is still frozen. The melted buttercream will soften the frozen part, while the frozen buttercream cools and hardens the melted part, and the result is the perfectly creamy mixture you had before it was frozen.
If the mixture seems slightly curdled, itís still too cold. Just take out about a half a cup, melt it, and add it back into the mix. This should warm it up to a good consistency. Hope that helps!
Here’s a link to the page: http://bravetart.com/recipes/swissbuttercream