Will the daughter's dress be all-white, or have some color to it?
Sometimes the name or crest/emblem of the cotillion will start you off on ideas. Or maybe the family's crest?
When I hear "cotillion", I think of opera-length gloves, pearls, and magnolia blossoms.
Blown sugar "bubbles" could be a starting point.....
HTH
How many people does it need to serve? This is where you could use some interesting shaped pans. I think a stacked comma shape doesn't really scream wedding as much. It also creates that area inside the curve that could be used for more highlighted decorations.
You could also do a topsey-turvey cake. They can be whimsical or classic.
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