If you want butter for quick breads and things (the baking process), pureed AVOCADO works BEAUTIFULLY - it has a rich buttery flavor and is PERFECT, so does applesauce with some oil (i usually use the same amount of applesauce for the oil/butter the recipe calls for and then add 2 tbs of sunflower/canola/whatever oil), and then of course for baking and such, u can also use coconut oil.

I've learned a lot about vegan baking, especially vegan substitiues, from Post Punk Kitchen. They have it down to a science and make it easy.

http://www.theppk.com/vegan-baking-the-post-punk-kitchen-shows-you-how/