Did you use spackling of joint compound? There is a difference and you may have better luck with joint compound. To change the texture try adding either white glue, water or both. Does your frosting need to be flexible or just look puffy? Does it get handled a lot? If it doesn't get handled much you may have good luck with just using frosting. It will get crusty over time and I wouldn't ever try eating it, but it might be worth a shot. You may be able to "seal" the frosting with a layer of white glue or "sculpt or coat" which you can find on Rosebrand's website.

PLEASE! I need help with "frosting"

Can anyone please help me find a product or combination of products that will make a giant cupcake "foamy,puffy,frosting"? I have tried several things such as whipping light spackling with a hand mixer and then piping it out, have also tried spray expandable insulation, but just can't seem to get the right consistency I am looking for. If you have ideas or know something that works, I would REALLY appreciate it! Thanks ahead of time!

Additional "frosting" ideas

Another "frosting" for prop cakes is Paintable Bathroom caulk. It can be airbrushed or dyed before applying. It will pipe through a pastry bag, although roses won't get as crisp an edge as real frosting. Will not crack or break at all if dropped, and if dyed before applying can be washed if it get's dirty. If you are trying for a fondant look, apply the caulk as smooth as possible, give a few minutes to start to firm up and using water and the palm of your hand for edges, smooth caulk until desired look. Make sure you give the caulk a chance to dry slightly between smoothing passes so it doesn't just wash away. I have had great result with this technique and the cakes last "forever".