Hi, Little Lottie!
Welcome! Looking forward to hearing about your baking!!!
It’s funny you ask this at this time, because there was just recently a post that tangented-off to just this topic!
I’ve found that I don’t have to rebeat Neoclassic buttercream, but here’s some info from Julie from this (http://www.realbakingwithrose.com/index_ee.php/forums/viewthread/3120/) post that’s really detailed and helpful on this point:
Here?s my take on this: Some batches of buttercreams seem to do well without the re-beating, and some definitely need it. I?m not sure I?ve totally worked out when it needs it and when it doesn?t, but I think it?s related to additions. If you have a lot of water (creme anglaise, fruit puree, etc) in the additions, they will turn spongy more quickly and then need re-beating.
In my experience the silk meringue needs re-beating 100% of the time (and this is what Ninuh is using), while vanilla mousseline (with only extract as the addition), almost never needs rebeating. I?ve only made white chocolate mousseline once, but it did need rebeating (maybe because I used cocoa butter and vanilla, so there was no lecithin like real chocolate?).
I made a batch of pineapple neoclassic buttercream but goofed on making the puree by straining it, so that the resulting puree was more watery than the recipe called for. When I added the full cup that the recipe calls for, it broke. I was able to fix it with a little rum (Rose says that alcohol helps the emulsion), but the buttercream turned spongy pretty quickly compared to others and needed re-beating several times while I was frosting and piping on the cake.
Sponginess can be a problem if you take too long to smooth the frosting that is already on the cake. If it has turned spongy, it will deflate and soften when you spread/smooth it and will look darker than the buttercream that hasn?t yet turned spongy. Then you either have to refrigerate it and spread a new layer of frosting on top, or scrape it off, hope there aren?t too many crumbs in it, re-beat, and start over.