I only use buttercream with my fondant. I have learned through trial and error and online advice the following. Fill your cake and crumb coat with buttercream. I let it settle for a couple of hours. This ensures the cake has shifted/settled. This stops the moving/sagging/air pockets in your fondant later. I like to roll my fondant out to the correct size before I put my final coat of buttercream on my cake. Basically it is just another thin coat of buttercream. This secures the fondant in place. I have not experienced any more fondant problems caused from the settling of the cake. Good luck!!!
Party Cakes
