Is there a way to get a frothy/bubbly icing/topping to put on top of a cake?
I'm wanting it to look like a cup of cappacino, frothy coffee
I'd either make whipped cream or Italian meringue.
You might try doing some Bubble Sugar. It's not icing but more like hard candy that when dried has holes in it - which can be interpreted as "steam" coming from the coffee - just insert it into the top of your cake. Check out http://candy.about.com/od/hardcandyrecipes/r/bubblesugar.htm
Site gives you ingredients and "how to". Can make with granulated sugar or Isomalt. Good luck.
Thanks for the ideas, I'll have a practise and see what works best