Air pockets can also form when filling the cups. I usually thump my pans flat on the counter 20 or 30 times before putting into the oven, that almost always takes care of any rogue bubbles! (I do this for regular cakes also!)
Yep, mixing too long AND/OR too fast can cause air bubbles. If you mix at a high speed, try taking it down a level or two. If you mix a long time, try mixing just long enough to be sure everything is incorporated.
And the tip by DelightsByE is excellent! I do that too, and it always helps get rid of air bubbles...if it's a smaller or lighter pan, you'll have to thump then down onto the counter, but if it's a larger or heavier pan, you can hold the pan several inches above the counter and just drop it several times.
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