Preheat oven to 350ºF and lightly grease with butter flavored cooking spray a 9” x 5” loaf pan; set aside. In a medium bowl, whisk together flour, baking soda, baking powder, brown sugar, cinnamon, and salt; set aside.
In a large bowl, beat together mashed bananas, butter, eggs, milk, sour cream, and vanilla extract. Add dry ingredients to wet; process until just moist. Add pecans and fold gently to combine.
In a small microwave-safe bowl, place chocolate and process for 1 to 2 minutes on “high” or almost melted. Remove from microwave and cool slightly. Add 1 cup of banana batter into chocolate and stir until well blended.
In the prepared loaf pan, spoon 1/3 banana batter and alternate with half chocolate mixture, finishing with banana batter. Swirl batter together with a knife. Transfer bread in the preheated oven and bake for 60 to 70 minutes or until a cake tester inserted in the center comes out clean. Remove from the heat and let cool in pan for 10 minutes before unmolding.