The mayonnaise in place of the butter makes the bread toast evenly and gives a similar yet different crispiness. This is how they made it on the grill at a diner I used to frequent, the kind where the grill is behind the counter and you can watch. CAUTION: using low fat or no fat mayo or even miracle whip will change the outcome of the final product, the first 2 contain water and the second sugar that does not act in the same manor as an oil or fat sub.
