I nearly keeled over when my aunt told me her delicious light chocolate cake has mayonnaise in it. Do you use it in place of, or as well as, butter and eggs?
This practice became popular during World War II when certain ingredients were scarce. You use mayonnaise (not sweet salad dressing) in place of oil and eggs. Leave out the eggs and add as much mayonnaise as the recipe specifies for oil. Your cake should be light and fluffy. Good luck!