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If you accidentally burned the outer edges of your cake, you can carefully cut them off with a knife. Make sure to trim the bottom of the cake, as well, since that area will be affected the most. Though your cake will be smaller, once it's covered in frosting no one will know what happened. As long as the cake isn't burned throughout, it should taste just fine once the burnt areas have been removed. To make frosting easier, place the cake in the freezer for about 30 minutes.
Add butter
Sometimes if a cake is overcooked it won't necessarily "burn," but will come out extremely dry. Dry cakes can be covered in frosting to help moisten them, but an even better trick is to spread melted butter on top of the cake before frosting it. The butter will melt into the cake giving it a moister, less crumbly texture."
I usually do this when I accidentally burn my cake! (Sheknows.com)