Preheat your oven to 180C. Line your loaf pan with baking paper.You may also choose to grease your pan first.
In a medium bowl, whisk together the flour, baking soda + salt.
In a separate bowl, use an electric mixer and mix butter and sugar for about 2 minutes. Add eggs one at a time and continue to beat well. Add in flour and mix on low speed till combined.Scrap down sides of bowl and ensure all is incorporated. Add the mashed bananas + sour cream + vanilla essence and beat till all mixed well.
Now to melt the chocolate. You may use the microwave. Place the chocolate pieces in a microwavable bowl and microwave for 20 seconds. Check and see if it needs to go further add another 10 seconds. When it's about 80% melted, remove and keep stiring it will go on to melt completely. Add cocoa powder and stir well. Now add one cup of the plain batter into this mix.
So now you will have two batters - chocolate and plain. Add the batters alternatively into the loaf pan. Swirl the batters around with a knife or chopstick 5 times. (The fun part for the kids!).
Bake for 50 minutes - 1 hour. Please check as every oven is different. For me this loaf cake was done after 50 mins. Once done, remove loaf pan and let cake cool in pan for a while before removing. Once removed, let it cool completely before slicing.