Our penchant (here in the U.S.) for topping baked sweet potatoes with marsh,allows should teach us that one really can't use them for cake icing. Problem is, although you can melt them readily and then pour on the ake, when it cools and especially whe it dries, it gains the texture and composition of very sticky cement. It will be nearly impossible to slice and will probably come off the cake in one very large marshmallow with a somewhat familiar crumbly cake coating underneath stuck forever to your knife.. Dogs won't even eat since most canines have enough brains not to put something in their mouths that will be there until the undertaker comes... I'm just saying...
(Read the next "Useful Tip" following the "cake icing" one... something abou "edible glue"... Pretty much sums it up...)