Preheat oven to 180°C / 360°F. Then grease and line a 1x4in loaf pan or two 5x4in pans with parchment paper long enough that it leave an excess hanging out at the sides. Set aside.
Peel the bananas and place 3 of them in a bowl. Mash the bananas using a fork or electric hand mixer until mashed but still lumpy. Take the other banana and cut into thin slices and set aside.
In a small bowl, combine milk and lemon juice to make buttermilk. You can also use buttermilk if you have it.
In a large bowl, mix the sugar and oil using a whisk or electric mixer. Add the eggs one by one and vanilla extract and beat until the mixture becomes smooth.
Next, pour in the buttermilk and mashed bananas to the mixture and whisk until well blended.
Add the flour, baking soda and salt and mix again until well blended and smooth.
Lastly, add ½ cup chopped walnuts, ½ cup chocolate chips, and the banana slices to the batter. Fold them together using a spoon or spatula until well distributed.
Pour the batter into the prepared pan. Tap it several times to release trapped bubbles and even the batter out. Place the pan at the middle rack and bake for 40-50 minutes or until an inserted toothpick comes out clean.
take it out of the oven and let it cool down enough to handle. Lift the excess parchment paper on the side to lift the banana loaf bread out of the pan.
Slice and serve while still warm.