Browse the internet for viable ingredient substitutions and you'll see suggestions of replacing cream with evaporated milk, or white flour with black bean purée, or two eggs with one banana, or butter with avocado, or canola oil with Greek yogurt... the list goes on (and images of cake disappointments flash through my mind).
While cooking likely leaves room for experimenting (and for covering up mistakes), baking is riskier: Substitute black bean purée for all-purpose flour (or a mashed avocado for a stick of butter) in your cookie dough, and I can almost guarantee you're not going to come away with a product you'll want to serve to friends.
So before you substitute willy-nilly, ask yourself: Will the swap alter the fundamental structure and composition of the final product? More specifically...
In simpler terms: Is the ingredient that's being added similar in fat/sugar/consistency/structure/acidity to what it's replacing? Tahini and peanut butter, for example, are viable for swapping: They both have 4 grams of fat per tablespoon and a similar viscosity (of course, you'll have to keep the flavor differences in mind, too). Tahini and avocado are not.
Alice Medrich warns, as general rules, against subbing liquid fats for butter; liquid sweeteners for sugar; acidic ingredients (buttermilk, lemon juice, vinegar) for milk or water; or gluten-free flour for over 50% of wheat flour without careful consideration.
Have you had success with crazy ingredient swaps that actually worked? Tell us in the comments below!