1. Preheat oven to 375°.
2. Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour and the next 5 ingredients (through salt) in a large bowl; stir with a whisk. Make a well in center of flour mixture.
3. Combine yogurt, banana, oil, vanilla, and egg in a small bowl; stir until well blended. Add yogurt mixture to the flour mixture in large bowl, stirring just until moist.
4. Pour the batter into a 9-inch square metal baking pan coated with cooking spray. Bake at 375° for 20 minutes or until a wooden pick inserted in the center comes out clean. Cool cake in the pan 10 minutes on wire rack; remove from pan. Cool completely on wire rack. Serve with whipped topping.