Preheat the oven to 160°C. Prepare a loaf tin with baking paper in it.
Melt the butter and whip it with brown sugar and cinnamon until the consistency gets creamy (3-4 minutes).
Add eggs, vanilla extract smashed bananas, and sour cream. Mix with a whisker until it's well combined.
Add flour, baking powder and whip until the flour disappears.
For the cinnamon swirl - mix in one bowl melted butter, brown sugar, and cinnamon. The main ingredient is melted butter, so keep on adding brown sugar until it gets really thick. Add cinnamon and mix again.
Into the loaf tin add a part of the banana bread mixture and a part of the cinnamon swirl mix. In the end, you should end up with layers of banana bread and a cinnamon mix. In the end, take a knife and move the batter around so the swirl can come to life. Sprinkle some chocolate chips over the top and bake for 1 hour - 1 hour and 15 minutes, or until the toothpick comes out clean.
Let the banana bread cool completely before cutting it.