21 minutes ago, ElsieD said:
That's one of the many things I like about this forum. There's always something to learn.
I can remember many many years ago in a Cook's Illustrated, a black and white photograph showing over-creamed sugar and butter for cookie dough. I don't know the science behind it but my suspicion is that it something like a broken emulsion when the butter fat breaks away from the water. One of our SSBs will be able to explain it I'm sure.