I found this receipe on receipezaar. It has maple syrup rather than brown sugar and you can definitely taste the maple sweetness. I'm listing the nutritional data supplied with the receipe at the end, I'm not sure how to calculate % of calories from fat but believe the fat is low enough for this to be safe.
Ingredients: 2 large sweet potatoes, cooked and mashed 1/2 cup skim milk (I subbed rice milk) 1/2 cup maple syrup 1 tsp vanilla 1/2 tsp nutmeg 2 eggs (I subbed 3 egg whites)
1. Boil sweet potatoes until soft (I peeled and cubed first) 2. Mash potatoes in a large mixing bowl 3. Combine mashed sweet potatoes with eggs, syrup, vanilla, milk and nutmeg. Mix well. 4. Spray 9-inch pie pan with non-fat cooking spray 5. Pour mixture into the prepared pan and bake at 350 degrees for 1 hour or until knife comes out clean