Preheat oven to 325℉. Line 2 8x4 inch loaf pans with parchment paper.
In a large bowl using an electric hand mixer, beat together the butter and sugar until light and fluffy. Beat in the eggs. Next, beat in the sour cream, bananas and vanilla until smooth. Transfer 1/2 of this mixture to a medium bowl. To the original, large bowl sift in 1¼ cup flour, 1/2 tsp cinnamon, 1 tsp baking soda and 1/4 tsp salt and mix until JUST combine.
To the second, medium bowl sift in 1 cup of flour, cacao powder, 1/2 tsp cinnamon, 1 tsp baking soda, and 1/4 tsp salt.
Scoop batters into prepared pans, 1/2 cup at a time, alternating vanilla and chocolate. Use a butter knife or skewer stick to swirl the batters. Bake for 55-60 minutes or until toothpick comes out clean. Allow to set in the pan for 10 minutes, then transfer to a wire rack to cool completely.