Preheat oven to 400 degrees. Spray 12 muffins cups with nonstick cooking spray. Combine flour, sugar, baking powder, baking soda, cinnamon and salt in a large bowl. Combine oil, water, eggs, and sweet potatoes in another bowl.
Add wet ingredients to dry; stir just until moistened. Spoon batter into muffin cups.
Bake for 20 minutes, or until done. Cool in pans on a wire rack for 5 minutes. Serve warm with butter.