Make That Cake has a photo tutorial on covering an extended tier square cake in fondant. Since it’s so tall, she used the paneling method but you can barely tell the seams are there at the end: https://www.facebook.com/photo.php?fbid=365988576833004&set=a.306056346159561.63975.304575132974349&type=3&theater
I haven’t done one myself, but from what I hear it’s important to ganache the corners perfectly and using either the upside-down and/or Australian methods of ganaching really help with not having droopy/rounded corners. Then be careful not to roll the fondant too thinly and SECURE the fondant to the top edges/corners of the cake by pressing with your fingers ASAP so that the weight of the fondant doesn’t cause it to rip at the edges. She hasn’t really sharpened the edges, but here’s a video of covering a square cake in fondant: http://www.youtube.com/watch?v=lyRy2QX2wKo
To get a really perfect sharp edges on your cube, you need to get THICK sheets of acetate (400 microns or thicker is ideal), cut 2 approximately palmsized rectangles out of the acetate, then use that to get super sharp edges on your fondant. There’s a little tutorial about it here but I can provide more explanation if it’s unclear still: https://www.facebook.com/notes/cakes-by-ade/sharp-edgesmy-how-to/256894157666002
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