Pre-heat oven to 350 degrees and butter and lightly flour a 9x5" metal loaf pan.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
Combine chocolate chips and walnuts in a small bowl. Add 1 tablespoon flour mixture to bowl and toss to coat.
In the bowl of an electric mixer or in a large bowl with a hand mixer, beat butter until light and fluffy, 2-3 minutes. Gradually add in sugar, beating until well blended.
Beat eggs into mixture one at a time. Add mashed bananas, lemon juice, and vanilla extract.
With the mixer on low speed, beat in flour mixture until just combined, taking care not to over-mix.
Pour about ⅓ of the batter into the loaf pan. Sprinkle with half of the chocolate chip and walnut mixture. Pour an additional ⅓ of batter over. Sprinkle with remaining chip/nut mixture and top with remaining batter.
Bake banana bread for about 55-60 minutes, until edges are turning golden and a toothpick inserted into center comes out clean. Turn out onto a rack and cool.