I was having the exact same problem when I started cake decorating. I would use a crusting buttercream under my fondant, but as soon as I put the fondant on the cake, the corners would round out. I've found that using ganache under my fondant works MUCH better. I can get crisp, clean corners with ganache, and it doesn't settle under the weight of the fondant. You can totally tell by looking at my pics which cakes are BC and which are ganache
I agree with yummy, but I've had some luck with buttercream, not super crisp. I use two fondant smoothers... I hold on upright on the side of the cake and have it halfway up and then with the other, I gingerly work the fondant on the top of the cake over to the side fondant smoother to get a "corner". Still working on getting this down but it kinda works. I also "pinch" the edge too to make it a little more sharp.
I've also found that it is easier to achieve sharp edges if you roll your fondant thinner (between 1/8" to 1/16"). Both ganache and SMBC will work great under fondant.
check out this youtube vid:
http://www.youtube.com/watch?v=imfDvvSZn5I&feature=relmfu
HTH
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I haven't tried ganache, but SMBC just out of the fridge works great!
( I mean the cake crumb coated in SMBC right out of the fridge.)
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