What sort of cornice is it????
Is it plasterboard coving or a fancy solid design one?
If its plasterboard coving then its simple to do, just cut your 45 degree angle and cut the back of the coving out where you see the exposed plaster, its kind of hard to explain but try a tester piece first before you cut the full length, you dont have to touch the coving on the solid wall that you're butting up to what so ever.
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