Preheat the oven to 175 degrees Celsius. Butter a 8x4-inch loaf pan and line the bottom with parchment paper. Set aside.
Place the butter and sugar in a large mixing bowl. Beat, using an electric mixer, on medium-high speed for 5 minutes until the mixture is pale, light and fluffy. Add in the mashed bananas and beat on medium speed for another 1 minute. Add in the beaten eggs and beat on medium speed for another 1 minute. Add in the buttermilk and beat on low speed until just combined.
Combine the flour, baking soda, baking powder, cinnamon, salt, walnuts and chocolate chips in a small bowl and stir to mix them together.
Add the flour mixture into the batter and stir with a large wooden spoon until just combined (until no flecks of flour is visible).
Transfer the batter into the prepared loaf tin and spread it evenly with a spatula or the back of a spoon. Tap the loaf twice on the table counter to get rid of excess air bubbles and bake for 45 to 55 minutes until a skewer inserted into the center of the loaf comes out clean. (Check the loaf after 30 minutes, if the top is getting overly browned, tent it with a piece of aluminium foil.) Let the loaf cool completely on a wire rack.
Turn the loaf out onto a serving plate. Slice and serve!