when doing a fondant cake how do you make the really sharp crisp edges around the top and corner of the cakes. i keep getting rounded edges. and how to get then with buttercream. it seams like i spend an hour just trying to smooth out buttercream where it is nice and pretty but i still get rough spots and rounded edges
Have you tried using chocolate ganache instead of buttercream. That is what I use when covering a cake with fondant because I too like crisp corners and edges.
This blog might help. I know people use the same method with buttercream and/or ganache.
Here's a buttercream tutorial for square cakes: