The main ingredient in mayonnaise is eggs, along with oil. You can substitute mayonnaise for eggs in cakes, and several recipes have been developed that make this substitution. It's best to substitute mayonnaise in cakes with a stronger flavor, such as a chocolate cake or a spice cake. This will mask the taste of the mayonnaise. Using mayonnaise instead of eggs produces a very moist cake with a rich flavor.
Hellmann's Mayonnaise popularized the idea of mayonnaise instead of eggs in cakes in 1937. The cake became more popular during World War II, when sometimes scarce supplies of eggs and oil caused housewives to search for alternatives, but Food Timeline.org reports that mayonnaise cake recipes appeared in magazines and cookbooks as early as 1927 and remain popular today. Hellmann's still publishes a mayonnaise cake recipe on their website, though the current recipe calls for three eggs in addition to one cup of mayonnaise.
Alice Henneman, a registered dietitian with the University of Nebraska-Lincoln Extension Service, advises substituting 2 tablespoons of mayonnaise for each whole egg called for in your recipe. Most recipes use a ratio of 1/2 to 1 cup mayonnaise for every 2 cups flour called for in the recipe, and no additional oil or eggs.
Chocolate Mayonnaise Cake
The original Hellmann's mayonnaise cake was a chocolate cake made with cocoa, and is still the most popular variation of mayonnaise cake found online, whether made with a boxed cake mix or from scratch. The mayonnaise stands in for both the eggs and the oil in these recipes. Other variations add spices and fruit such as raisins.
Other Egg Substitutions
If you're out of eggs, or don't wish to use eggs and don't have mayonnaise, you can use other ingredients as a substitute for eggs in your cake. For each egg called for in the recipe, add ½ tsp. baking powder, 1 tablespoon vinegar and 1 tablespoon water or other liquid. Conversely, you can add 1 tablespoon soy flour and 1 tablespoon water for each egg.