Preheat oven to 180 degrees Celsius. Line a small loaf pan (about 900g)
Add the butter, oil and sugar to a large bowl. Use a hand mixer and mix until it's pale in colour. About 2 minutes
Mix in the vanilla essence
Beat in the eggs until well combined. Add the mashed bananas and buttermilk,mix. Mix in the bicarbonate of soda and flour until combine. Scrape down the sides of the bowl as you mix
Place half the batter in a another bowl (this step is optional). Add the cocoa to one half of the batter.
Grease and line a loaf pan. Pour one third of the plain batter in one corner of the pan and one third of the chocolate next to it. Repeat two more times. Use a skewer and swirl it through the batter. Sprinkle the chocolate chips on top
Bake at 180 degrees Celsius for 45 minutes to an hour. Or until a toothpick is inserted and comes out clean
Allow to cool for 10 minutes before removing it from the pan. Allow to cool further on a cooling rack