The first concern most people have when they consider this substitution is flavor. Mayonnaise has a distinctive taste — tangy, creamy, slightly eggy — and the idea of that flavor in a chocolate brownie is genuinely off-putting. The reassuring reality is that this concern turns out to be completely unfounded in practice. The chocolate and sugar in a brownie mix are powerful enough flavor compounds that they entirely dominate the final taste. The brownie does not taste like mayonnaise. It does not taste like eggs or vinegar. It tastes like a very good chocolate brownie — richer and more intensely chocolatey than you might expect, with a moist, almost fudge-like interior and a slightly denser texture than the standard version.
The small amount of vinegar or lemon juice present in most commercial mayonnaise — which gives it its characteristic tang — actually performs a useful function in baking. It slightly acidifies the batter, which can deepen the chocolate color and subtly enhance the cocoa flavor in a way that is not identifiable as sourness but registers as added richness and depth.
How to Do It
The substitution is as simple as it sounds. Use the same volume of mayonnaise as the recipe calls for in vegetable oil — if the box asks for a third of a cup of oil, use a third of a cup of mayonnaise. Use plain, full-fat mayonnaise without any added flavors, garlic, or other seasonings. Low-fat or reduced-fat mayonnaise can be used but will produce a slightly less rich result because the fat content of the finished brownie is lower. Add the mayonnaise to the mixing bowl exactly as you would the oil — stir it in with the eggs and water until the batter is smooth and uniform, then bake according to the package instructions. No adjustments to temperature or timing are necessary.
Some bakers find that the batter is slightly thicker when mayonnaise is used instead of liquid oil, because mayonnaise is a semi-solid rather than a pourable liquid. If the batter seems too thick to spread easily in the pan, add a tablespoon or two of water and stir to loosen it. This is typically only necessary if you are making a particularly thick slab of brownies; for a standard 8-by-8 or 9-by-13 pan, the batter usually spreads without any adjustment.
