I'm making one for Christmas Day, but today is Monday and Christmas is Sunday. It only needs to be frosted; will I be OK or should I find an excuse to eat it early?
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I would do it closer to the day. In my experience genoise is already a dryish cake, so unfrosted it risks drying even more. Frosting would seal in a little moisture, but frosting early risks its appearance, which is sort of the whole point of it. It's a finicky sort of cake.
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Aren't you filling it as well? If you make it this early it will be "old" by Sunday. So if you want it for Christmas dinner, Friday should be your optimum day for making it.
I agree with Chef June. Am making one as well but will be brushing the genoise with a liqueur such Chartreuse or Kirschwasser so will likely make a bit earlier than Friday but not much earlier than that. Most pastries soaked with a liqueur like to rest, in my opinion, for a day or so before serving.