I am trying to make a square cake that looks like a tiffany's box. Every time I have practiced the corners droop down. Does anyone have a any suggestions on how to make the corners not sag down??

I would chill the cake after crumb coating it and then when you butter cream it again be sure and use a straight edge up against the cake on the side where you are bringing icing to give straight flat edge...I can't explain better, try and find you tube video show icing square....then chill it again , and then cover with fondant. hth