I did see on a blog a person's experiment with putting MMs in cupcakes (she was trying to make a s'more cupcake). She used the large MMs, put one in each CC, and it puffed way up, then shrunk back down. She said that while they didn't look very good, they tasted very good.
So maybe try your cake with the miniature MMs and see how it works. But be prepared with extra ingredients in case you are not happy with the results and need to make another cake sans MMs.
Good luck and let us know how it turns out.
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