Replacing the butter in a recipe for health reasons, flavour reasons, or because you ran out is a bit of an art and your butter substitute choice can end up being anywhere between the best thing since sliced bread and the downfall of humanity.
Taking four years of home economics classes and university food science courses has taught me many tricks of the cooking/baking trade and one of the most useful is how to substitute one ingredient, like butter, for another.
Applesauce
Applesauce is a pretty good bet when you want to use less fat in a recipe, and it's definitely one of the most commonly used alternatives. Since it replaces some sweetness of sugar and some properties of eggs, applesauce works pretty well in recipes, though it will be best if used to replace only half the butter.
Avocado
Pureed avocados can be used in place of all of the butter in your recipe. While the fruit is high in fat, it's also a great source of many vitamins. Mashed avocados are also sure to make your dessert extra creamy, as long as they're ripe.
Sweet potato
Sweet potato (an all-time staple in my kitchen) has even more potential than we all thought. Sweet potato can be used in place of butter in a recipe, though it's probably best to only substitute 50% of the fat in your recipe, otherwise the texture may be off.
Beans
I know this sounds straight up crazy, but beans actually work well in nearly all baked goods (with the exception of pastries) and they make things a little more healthy if your'e feeling down about eating a third muffin. You can use basically any kind of cooked bean (pinto, kidney, black, etc.) in place of all butter in a recipe. For the sake of consistency, I would blend/puree them before using.
Pineapple
Pineapple can be used in place of about half of the fat in a recipe. Although it isn't the best alternative if you want a neutral flavour, it adds moisture and sweetness, which work very well in quick breads like muffins, carrot cake, and cupcakes.
Bananas
Bananas are a great substitute because they have they have a similar function to both eggs and butter when baked. Bananas can be used in place of most or all of the butter in a recipe, it just might take a few attempts before the ratio is just right. For the best results, I would make either puree bananas or whisk them (like you would an egg) before using.
Pumpkin puree
Pumpkin puree can be used in place of all butter in a recipe, but you need to make sure you reduce the amount you use by 25%. If your recipe calls for 1 cup of butter, use 3/4 of cup of pumpkin puree in its place. Pumpkin puree makes baked goods moist and flavourful, so it's a win-win, and not at all basic no matter what anyone says.
Greek yogurt
High in both protein and fat, Greek yogurt can replace half of the butter in a recipe. This works well in cookies, muffins, and pretty much anything except pastries. Looks like Greek yogurt is good for more than just breakfast.
Sour cream
Using sour cream in a recipe may seem kind of gross, but it's actually just like using Greek yogurt. You can use it to replace half of the butter in your recipe and it's a great way to use up your sour cream before it goes bad.
Coconut oil
Replacing butter with coconut oil won't actually bring the fat content of your dish down, but it might add a nice flavour. Coconut oil can replace all of the of butter in a recipe, and it can be used in nearly all recipes.
Nut butters
The high fat content in nut butters makes them a good substitute for butter in most recipes. Their protein content also adds more structure to a baked good, which makes them great for vegan baking.
Maybe you want to be healthy and reduce the fat content of a baked good, or maybe you're like me and tend to run out of butter mid late-night baking session. Whatever your reason for using a butter substitute in a recipe, check your pantry for some of these ingredients before hitting the store.