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.