Creaming is the process of creating air pockets in butter; the mixer uses the crystals of sugar to create small holes in the butter, that are expanded by the cake's leavener when the cake is baked. Overcreaming does a couple of things: it overheats the butter, making it soft, and it creates so many holes that the structure that's created can't hold itself together, and the weight of the batter collapses the cake, creating the streaks you saw in the picture. Susan

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