Substituting Mayonnaise in Recipes
So let’s say you’re baking a cake. You’re putting in eggs, right? And you’re using margarine, which is going to melt (a.k.a liquefy, as in ‘oil’) in a hot oven anyway, right? So what’s the problem here?
There is no magic to using margarine in a recipe. Any fat in cake recipes (butter, margarine, oil, shortening) adds to the tenderness and moistness of the crumb by preventing flour from forming gluten [which is not an issue on Pesach. So you learned an extra something. Shoot me.], which would make it chewy and doughy. Margarine/oil/fat doesn’t not add to the structure of the cake (flour, eggs), the rise (baking powder/soda) or the flavor (sugar/vanilla/cocoa). Ignore the stick of margarine behind the curtain…
The “blech” factor in substituting mayonnaise for margarine comes from the typical additions of vinegar and/or lemon juice, mustard, and salt. And because you most likely associate mayonnaise with tuna salad and cole slaw. Or maybe you’ve left out mayonnaise on a hot day and it turned disgusting. All irrelevant. Eggs. Oil. No flavor
Mayonnaise substitutes equally for margarine, as both are about 80% fat. And even though mayonnaise has more volume, you can substitute it 1:1 for margarine in a recipe, and because it’s less dense, you’re actually using less fat than if you used margarine, and you’re cake is going to be that much fluffier!
And, as an extra added bonus, now you know how to make your own mayonnaise for tuna/egg/potato salad, cole slaw, French fries, aïoli…
I did a fair amount of checking, and I couldn’t find anywhere, in halacha books, cookbooks or even Wikipedia, that says you’re obligated to tell anyone you used mayonnaise in your baking.