Okay I made a Cheerios box cake with the Hungry Caterpillar (grooms cake...don't ask...I haven't a clue!) that I will upload in a sec....anyway I did my buttercream corners nice and sharp. However when I put my fondant on top, it seemed the buttercream got soft and didn't stay sharp to mold the fondant over. Or maybe the fondant has to be paper thin to get the sharp corners???
Wish I could help...here's a bump!
Refrigerate the cake both befor eand after putting the fondant on... it'll keep the buttercream firm until the fondant has time to "set" and your corners will stay nice and crisp.
I do refrigerate but by the time I'm done rubbing the fondant smooth, the corners are rounded.