Preheat oven to 325 degrees F and grease an 8x8 baking pan with nonstick cooking spray and set aside.
In a small bowl, combine flour, cocoa, baking soda and salt and mix well. Set aside.
In a separate, larger bowl, add butter, sugar, sweet potatoes and mix well. Stir in egg and vanilla and mix until combined well. Fold in flour mixture and mix until just combined.
Spread batter into prepared pan and bake for 30-35 minutes or until toothpick inserted into the center comes out clean. Cool completely before slicing and serving.