Preheat your oven to 325 degrees F. Spray a 12-cup bundt pan with non-stick spray and dust with flour, granulated sugar or cocoa powder. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and the sugars on medium-high speed until light and fluffy, about 2 minutes.
Gradually add the eggs, scraping the sides of the bowl with each addition, Mix for about 2 minutes on medium speed after all the eggs are added.
Add the mashed bananas and vanilla. Stir until combined.
In a medium sized mixing bowl, whisk together the flour, cocoa, baking powder, baking soda, and salt.
In a small mixing bowl, stir together the buttermilk and sour cream.
With the stand mixer on low speed, alternately add the the flour mixture and the buttermilk mixture, starting and ending with the flour. Mix just until the ingredients are combined.
Fold in the mini chocolate chips.
Pour the batter into the prepared pan and bake for 75 to 80 minutes. (You can also increase the baking temp to 350 F and bake for 50-60 minutes - either option will work for this cake).
Let the cake cool completely in the bundt pan before inverting it onto a wire rack or your cake stand. This helps prevent the cake from sticking to the sides.