Preheat oven to 350 degrees. Grease a 9 x 5 loaf pan and set aside.
Add butter to a small saucepan and cook over medium heat until light brown / caramel in color. 5-10 minutes. Make sure to keep and eye on it so butter does not burn. It also helps to use a light colored saucepan for this.
Pour the browned butter in a large bowl along with the sugar and vanilla, mix until combined.
Fold in the eggs one at a time then add in the bananas and sour cream.
In a large bowl combine flour, baking soda and salt.
Add the dry ingredients to the wet ingredients.
Scoop 1 cup of the batter into the bowl you used for dry ingredients. Add in cocoa powder and mix to combine.
Pour both batters into the prepared pan alternating between chocolate and regular. Use a butter knife to swirl.
Bake for 1 hour or until a toothpick comes out clean.
Remove from the oven and let cool in the pan for 20 minutes.
Take the loaf out of the pan and allow it to cool completely on a wire rack.