Just think abt the plane of cut


#1

Assume cake is uniform along its height and so the plane of cut should be parallel to the plane of the cake surface.


#2

I think you misunderstood the question.
“Consider a rectangular cake with a rectangular section (of any size or orientation) removed from it. Is it possible to divide the cake exactly in half with only one cut?”
But you are cutting it parallel to it’s upper surface.