How to Bake with Fruit Puree Instead of Fat  

Excerpted from Brilliant Food Tips and Cooking Tricks, by David Joachim David Joachim.
Simple Solution
Fruit purees can be used successfully to replace some of the fat in baked goods.
Drained applesauce, strained baby food fruit, or a puree of prunes and water all work well. You can also use a puree of water and any dried fruit, such as dried apples, apricots, or peaches. The quickest option: a product called fruit puree fat replacement, carried in most supermarkets. It's a combination of pureed fruits and lecithin, a natural emulsifier found in corn, soybeans, and egg yolks.
Homemade Fat Replacement
Cut 2 apples and 2 pears into chunks and place in a saucepan with 2/3 cup water and 1 tablespoon each lemon juice and lecithin granules (available in health food stores). Bring to a simmer, cover, and cook 40 minutes, mashing occasionally. Press through a sieve or food mill. Refrigerate up to 5 days or freeze up to 6 months. To use, substitute fat replacement for about two-thirds of the total amount of oil, butter, or shortening in baked goods, adding 1 to 2 tablespoons canola oil. For example, to replace 1 stick (1/2 cup) butter, use 5 tablespoons fat replacement and 2 tablespoons oil. Makes about 3 1/2 cups.
Prune Puree
Puree 1/2 cup pitted prunes (or equal amounts of prunes and dried apples) and 1/4 cup water in a blender or processor until smooth. The dark color and strong flavor of this fat replacement make it best suited to chocolate-based or heavily spiced baked goods. To replace 1 stick (1/2 cup) butter, use 1/3 cup prune puree. Makes about 2/3 cup.