1 1/2 cups all purpose flour
1 teaspoon baking soda
1 teaspoon baking powder
1/4 teaspoon salt
3/4 cup semisweet chocolate chips
3/4 cup walnuts, toasted, chopped
1/2 cup (1 stick) unsalted butter, room temperature
1 cup sugar
2 large eggs
1 cup mashed ripe bananas
2 tablespoons fresh lemon juice
1 1/2 teaspoons vanilla extract
Preheat oven to 350 F. Butter and flour 9x5x2 1/2-inch metal loaf pan. Whisk first 4 ingredients in medium bowl to blend. Combine chocolate chips and walnuts in small bowl; add 1 tablespoon flour mixture and toss to coat.
Beat butter in large bowl until fluffy. Gradually add sugar, beating until well blended. Beat in eggs 1 at a time. Beat in mashed bananas, lemon juice and vanilla extract. Beat in flour mixture. Spoon 1/3 of batter into prepared pan. Sprinkle with half of nut mixture. Spoon 1/3 of batter over. Sprinkle with remaining nut mixture. Cover with remaining batter. Run knife through batter in zigzag pattern.
Bake bread until tester inserted into center comes out clean, about 1 hour 5 minutes. Turn out onto rack and cool.