Mayonnaise is an effective -- though surprising -- replacement for butter in baked muffins and cakes. Simply add the mayo when your recipe calls for you to add butter. Real mayonnaise is an emulsion of oil and egg, two common ingredients in muffins and other baked goods. The replacement will produce similar results, but the mayonnise-infused muffins will have a richer flavor and more tender texture than those baked with butter. According to Hellman's Mayonnaise, butter and mayonnaise both contain the same percentage of fat -- 80 percent. Cakes are the same as muffins but cooked in a larger pan instead of muffin tins.
Replace the butter in the recipe with the same amount of mayonnaise. For instance, if your recipe calls for ¼ cup of butter, use ¼ cup of mayonnaise, which equals 4 tbsp. of mayonnaise.
Omit the eggs from the recipe. Replace each egg subtracted from the recipe with 3 tbsp. of mayonnaise. For instance, if your recipe originally called for one egg, delete the egg and use 3 tbsp. of additional mayonnaise for a total of 7 tbsp. of mayonnaise. This includes the 4 tbsp. you added to replace the 1/4 cup of butter, as mentioned in the previous step.
Bake as directed per your muffin recipe.