I've had this problem a few times, and I think it has to do with the butter and the meringue powder. Sometimes if you get your butter too warm it can make it do that and sometimes just adding the meringue powder has done it a few times to me...not sure why. But usually if I put it in the fridge for 15 min it usually helps. And usually this has happened when I'm making cakes for just the family and they say they don't think it's grainy at all. So if worse comes to worst, when they are eating the cake, I don't think they will notice.
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