"Either cakes or biscuits are delicious."
"Either cake or biscuit is delicious."
I've never heard "cakes or biscuit". I think if you were going to make one of the words plural, it would be biscuits.
"Either John or Mary is the best student." "Neither John nor Mary is the best student."
"Either boys or girls are better at maths. Or perhaps neither are better than the others at maths."
Student or Learner