1. Heat oven to 400 degrees F. Lightly coat 6-cup bundt or tube pan with nonstick cooking spray.
2. In medium-size bowl, stir together flour, baking powder, salt, cinnamon, baking soda and nutmeg.
3. In second medium-size bowl, with electric mixer on medium speed, beat together egg, sugar, mashed banana, oil and vanilla until smooth, about 2 minutes. On low speed, beat in flour mixture just until blended. Stir in walnuts. Spread batter evenly into prepared pan.
4. Bake in 400 degree F oven for 30 to 35 minutes or until wooden pick inserted comes out clean. Let cake cool in pan on wire rack 10 minutes. Unmold cake onto rack; let cool completely.
5. To garnish, place semisweet chocolate and white chocolate in separate microwave-safe small bowls. Microwave on high power 1 minute or until melted, stirring halfway through. Spoon chocolate into separate small plastic food-storage bags. Snip off end of each. Drizzle over cake.