If given a rectangular cake with a rectangular
piece removed (any size or orientation), how would you cut the
remainder of the cake into two equal halves with one straight cut
of a knife?

Solution

Find the centers of both the original cake and the removed piece. Cut the cake along the line connecting these two centers. As this line cuts the original cake and the removed piece in half, the remainder is two equal halves.

OR

Cut the entire cake in half horizontally (i.e. parallel to the table). This will get you two even halves.