I making a cake covered in buttercream frosting for my nieces b-day. I want to roll out the marshmallow fondant and cut out out shapes ( hearts and circles) and place these shapes onto the cake. How can I get the shapes to stay on the buttercream frosting cake?? How far in advance can I make these shapes?? How long will they be edible for?? Will they shrink OR turn rock hard. Please help ! Any suggestions or help would be great.

You will want to apply them while the frosting is still soft. I wouldn't make it more than 2-3 days ahead of time. I've personally had great luck covering the butter cream frosting with fondant and then applying the cut outs over that. I apply them with a semi stiff frosting. (It will dry fast that way and hold better). Good luck. Marshmallow fondant is always a hit. It tastes so good!

It doesnt dry rock hard, thats why I love it. well for me atleast.... and apply while the frosting is wet or if it crusted then brush a tiny bit of water on the fondant shape then apply.