Is the towel cake already made? Do the towels have a fragrance of their own? If they do, you might consider taking the cake apart and washing the towels with a very minimum of fragrance-free detergent, maybe an eighth of a cup or less, then adding white vinegar to the final rinse. Then do not use any sort of softener in the rinse or in the dryer. You do NOT want to add some other fragrance. (Adding the vinegar will make the towels fluffy and static free.)

As for adding buttercream fragrance, Yankee Candle has Buttercream. Link below is to the Buttercream candles. If link does not work for you, just go to www.yankeecandle.com, then click on "shop by fragrance," then "alphabetical."

I think if you stuffed unwrapped votive size candles inside the layers or core of the towel cake as you assembled it, the buttercream aroma will permeate the cake before the wedding.

Here is a link that might be useful: Yankee Candle Buttercream Fragrance