Sometimes I've gotten bubbles while baking and don't know exactly why since I use the same recipe each time. When I do get them, during baking while I check the peices, if I find bubbles, I poke them with a toothpick which helps the bubbles to go back down. But, since I usually frost each wall peice anyway (as you would paint a wall) - the bubbles dont' really affect me too much.
Hope this helps. Sorry I couldn't provide reason why the bubbles form....I'm sure it's trapped air of some sort...
Loreta
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