There are many reasons to replace butter in baking, and its not just to reduce fat grams. Butter is loaded with cholesterol, and additionally, many people are concerned about hormones in their dairy products, but buying organic butter can be a serious financial investment. In order to decide how you will replace butter in your recipe, you need to understand the purpose of butter in your baked goods.
In cookies and cakes, butter is typically creamed with sugar to add air to the recipe. This air adds structure to the recipe and also helps prevent all of the moisture from being lost in the baking process. As the flour is mixed in, the fat in butter coats many of the grains and helps to prevent gluten from developing as you mix the batter. Gluten is the substance in flour that makes breads chewy, and too much gluten development in your recipe makes baked goods tough. Pie crusts and crumble toppings use butter to promote a flaky, crisp texture by literally frying the flour and also creating little spaces in the layers as the butter melts.
Generally you can use shortening as a one-to-one replacement for butter, but it adds unhealthy transfat. Another option would be to use a transfat-free, but still full fat margarine, like Earth Balance or Canoleo. However, these options are still all high in fat.
Picking a lower fat or healthier option to replace butter in your recipe should depend on the purpose of butter in your baked good. For example, crumble toppings need a certain amount of fat to "crisp" up. Without fat, crumble topping turns into an unappetizing granola. Using a reduced fat margarine will work as a one-to-one replacement in this case. This should allow you to cut the fat in your crumble by 1/3 or 1/2.
Most cookie recipes that involve creaming butter and sugar need some amount of fat to retain their original texture. All-natural, no-sugar-added peanut butter is a good option for replacing butter in chocolate chip cookies, but will change the flavor. Using 1/2 butter or margarine and then adding soymilk to get the right consistency after adding the other wet ingredients works well, but sometimes can make cookies a little chewier than usual.
Oil can replace butter as well, but expect your cookies not to rise as well, and they may cook faster than usual, creating crispy edges.
Muffin and cake recipes are much easier to replace butter in. The more liquid in a recipe, the less vital butter is to the recipe. You might want to try a smaller batch of the recipe the first time you use a substitute. I've generally found that the following replacements work really well, particularly if you measure them just shy of the 1-for-1 replacement and add a teaspoon of oil. Also, don't over mix your recipe. The more you stir while adding the dry ingredients, the greater potential for over development of the gluten in the flour.
Good Fat Replacements For Baking:
* unsweetened applesauce - adds no flavor, but can make your baked goods chewier
* pureed prunes - prunes have a distinctive flavor, so unless you love the taste of prunes, only use them in recipes that have a lot of strong flavor, like chocolate or spice cake.
* fat-free yogurt (regular, greek, or soy) - adds protein to your recipe and can sometimes make recipes a little too moist.
* you can also mix these together to create a custom butter replacement
There are many ways to replace the butter in your recipes, but the most important thing is to experiment and decide the best combinations of butter replacers for your favorite recipe!