This hasn't happened to me until very recently. After covering a cake in fondant, and feeling totally pleased with it I placed it in the fridge. Within a few hours, I notice HUGE air bubbles forming on the sides of the cake and making distended places. I can pierce them with a pin, but the fondant is set and will crack or have a crater when the air is released. WTH????
I used to have that happen until I read from others here about the gas inside the baked cake and how I should let the cake sit overnight before icing and covering it. I haven't had a problem with that since. You will also get bubbles if you are working in heat and the cake is in the heat as the marshmallow part wants to expand, (kinda like a slower version of putting a marshmallow peep in the microwave) but if it's happening after it's in your fridge, I'd try baking the cake one day, letting it sit out overnight, then starting work on it.
I used to have that happen until I read from others here about the gas inside the baked cake and how I should let the cake sit overnight before icing and covering it. I haven't had a problem with that since. You will also get bubbles if you are working in heat and the cake is in the heat as the marshmallow part wants to expand, (kinda like a slower version of putting a marshmallow peep in the microwave) but if it's happening after it's in your fridge, I'd try baking the cake one day, letting it sit out overnight, then starting work on it.
I understand the part about letting the cake sit over night, I to have started that, but have I missed something about Marshmellows? Or are you refering to marshmellow fondant? Very confused!!! 
I'm sorry. Yes, I was referring to the marshmallowey, stretchy part of the fondant.
Ahhh, sorry, I got confused. 
I've never tried marshmellow fondant. I make rolled fondant. That makes sense tho...
well, I have been letting my cakes rest and actually wrapping them and putting them in the fridge before I level them or frost them. If it is condensation under the fondant, what do I do to aviod it?
Its under the buttercream...between the buttercream and the cake.
Just be sure when you are icing your cake you are icing it firmly enough to where there are no pockets of air...that's all I can tell you, I don't think anybody knows 100% how to avoid it....
Also make sure your buttercream is smooth enough to where there are no gaps between the bc and fondant.
Quote by @%username% on %date%
%body%