Eggs on their own can be used for leavening, glue, moisture, emulsification, fat, and foam. This makes them uniquely effective when trying to achieve specific textures – think the creaminess of a lemon bar, or the foaminess of a meringue. It also makes them very complicated to replace, so when you’re trying to replace eggs, you need to start with what purpose the eggs are serving in the recipe at hand. In many recipes, your best bet will be to mix and match various replacement methods.
When you need eggs to help your cakes and cookies rise, think about ingredients that have leavening agents naturally, such as beers and yoghurt. Presumably, buttermilk and sourdough starter could work too, but I haven’t personally given them a try. Non-flavored yogurt can usually replace eggs at a 1T to 1 egg ratio in recipes like pancakes. I recommend adding or upping the quantity of baking powder the recipe normally takes as well. Beers and ginger beers work well as a replacement by substituting liquid content in a recipe. For example, my favorite banana bread calls for water. I replace 100% of that water with natural ginger beer.
Many recipes use eggs as a way to hold ingredients together as a glue or emulsifier. Depending on whether your recipe is savory or sweet, you may have luck with bananas, mashed potatoes, chickpea flour, or flax eggs. Mashed bananas or grated zucchini work well in cakes and quick breads while mashed potatoes work wonderfully in fritters. Chickpea flour and flax eggs can work within recipes together with other methods and are also great options when you’re trying to get breadcrumbs to stick on something like fried chicken.
This is a trickier category. It can mean moisture like in a cake (try bananas, grated apples, or aquafaba), or it can mean foam as in meringue (I have heard amazing things about aquafaba, but haven’t tried it yet personally), or it can mean the chewiness of a chocolate chip cookie. Unfortunately, I still haven’t found a satisfying replacement for that chewy-crunch in a cookie, so let me know if you’ve found a winner!
Please note when testing out substitutions that the protein in eggs is often a major factor in the moisture of baked goods, so substituting for gluten free and egg-free can be very difficult as most gluten-free flours have lower protein content.