Preheat the oven to gas mark 3/325oF/160oC/140oC fan and line a 2lb/900g loaf tin with parchment paper. Add the butter to a microwaveable bowl and melt for 35 seconds or until fully melted.
In a medium bowl, whisk together the melted butter, vegetable oil and sugar. Mix in the eggs then the vanilla extract followed by the sour cream. Fold in the flour then pour half the batter into another bowl.
To one of the bowls, add the mashed bananas and fold it into the batter. To the other bowl, add the cocoa powder and mix until smooth.
Add a dollop of banana batter to the loaf pan then add some chocolate batter. Place the batter randomly in the tin to create a marble effect. Repeat this process alternating between both batters until all the batter is used.
Bake the loaf in the centre of the oven for 50 - 65 minutes. The time may vary so check every 5 minutes with a skewer once it has baked for 50 minutes.
Remove the loaf from the oven and allow to cool in the tin for 40 minutes then remove the loaf cake from the pan and place directly on the wire rack to completely cool before slicing.