Hmmmm, could you make a small *white cake* and using a round cutter - cut out the shape you need?
That way you would get the bubble effct as the white cake would have small air bubble texture when looking at it disected.
You could always freeze what you do not use - or make cake balls.
Or - fondant -
Once in the shape you need, you could poke little holes in it to give the impression of bubbles.
You could use a BRAND NEW babies comb sterilized of course before you use it.....to get all the little holes instead of doing them individually -
Might this be for St Patricks day? 
If so you could make grreen cake balls from the left over white cake- 
Bluehue
Hello! Marshmellows work very well. If your familiar with making mmf then you can complete the first few steps (water marshmellows) don't melt all the way, then pour into a greased cup or mold. After it dries remove the whole thing and place on top of cake. Hope this helps.............
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