This happens to me often. I bake, cool, trim, fill, level and refrigerate (filled and crumb coated with SMBC). Take it out of the fridge and smoothly ice with SMBC. Chill again overnight. Cover with fondant while completely chilled.

The only thing I can think is that when the icing comes to room temp under the fondant, the fondant doesn't have enough 'support' and cracks.

Please, please please help me out. I'm super frustrated!

It looks like an air bubble, do you brush cake with water and then cover with fondant.

looks like air bubbles under the fondant, too much build up can cause the fondant to crack

SMBC doesn't absorb corn starch or powdered sugar like ABC does - so whatever you use to roll out your fondant can prevent it from sticking fully to your SMBC. No matter how skilled you are at getting out air bubbles, new ones will appear.

3 ways to combat this:

1) after you chill your cake you can brush the surface of the cold buttercream with either water, shortening, butter or piping gel. I prefer shortening in case I need to pull off and re-cover if I make a mistake but some claim you will get the best results with piping gel since it works as glue.
2) roll your fondant with as little powder as possible
3) make sure you get as much powder off the side of the fondant that will be touching the cake before you put it on.

Hope this helps!

4.) Roll on a smear of crisco rather than PS or CS
5.) Purchase The Mat, which is food safe vinyl, and roll between both pieces.

HTH
Rae

This is extremely helpful. I will try everything you've suggested...thanks so much for the tips!

I will also make little pin prick behind the edges of my cutout pieces, gives the air a place to escape instead of building up behind. I make them in several places around the cake. Since I've started doing this I haven't had any blow outs.

Thanx for the tips y'all.