1. Heat oven to 350 degrees F. Grease and flour two 9 x 5 x 3-inch loaf pans.
2. Combine flour, baking soda, baking powder and salt in large bowl.
3. Blend banana, sour cream and vanilla in medium-size bowl.
4. Beat butter in large bowl until creamy. Gradually add sugar, beating until light and fluffy, about 3 minutes. Add eggs, one at a time, beating well after each addition. Beat in flour mixture alternately with banana mixture, beginning and ending with flour mixture. Stir in chocolate chips and nuts. Divide batter between pans.
5. Bake in 350 degree F oven 70 minutes or until toothpick inserted in centers tests clean. Let loaves cool in pans on wire racks 10 minutes. Turn loaves out onto racks; let cool completely.Toppings:
6. Melt butter in milk in saucepan. Pour into bowl. Gradually whisk in sugar until good drizzling consistency; add a little more milk if needed. Drizzle half over each loaf. Sprinkle nuts over each. Spoon melted chocolate in small plastic food-storage bag; snip corner. Pipe over loaves. Wrap tightly in plastic. Store at room temperature up to 2 days, refrigerate 3 to 4 days, or freeze up to 1 month.