Preheat oven to 325 degrees F. Grease two 9-inch loaf pans with baking spray. In a medium bowl, sift next six ingredients.
In a large bowl, using an electric mixer on medium, cream butter, sugars, and vanilla until light and fluffy. Blend in sweet potatoes, then eggs, one at a time. With mixer on low, add flour mixture in thirds; combine just enough to blend.
Spoon batter evenly into prepared pans. Sprinkle pecans over batter. Bake until cakes test clean when a skewer is inserted into centers. 70 to 80 minutes. Cool completely on wire rack.