Preheat oven to 350 degrees F. Combine flour, sugar, baking powder and salt in a medium mixing bowl. Whisk together egg, bananas, oil and vanilla. Add to flour mixture and stir until flour is moistened. Fold in walnuts. Spray 8-1/2 x 4-1/2 loaf pan with cooking spray. Pour batter into prepared loaf pan.
Bake for 35 to 40 minutes or until puffed and golden brown. A wooden pick inserted in the center should come out clean. Transfer loaf pan to a rack to cool for 10 minutes before turning out of pan. Serve warm or store in an air-tight container.