The icing doesn't have to crust to be used under fondant, but it does have to harden in the fridge. Otherwise the cake will be a lumpy mess and all the buttercream will ooze out the bottom when you are trying to cover it. I quit using buttercream under fondant. Instead I use white chocolate ganache. Ganache holds up so much better to the weight of fondant than buttercream.
11/5/14 at 7:08am