Preheat the oven to 350°F. Grease a 9x5-inch loaf pan with cooking spray. 

Add avocado, egg, and vanilla to a food processor or blender. Puree until smooth. Add mixture to a large bowl and stir in the mashed bananas. 

Add flour, cocoa, brown sugar, baking soda, and salt to the banana avocado mixture. Stir to combine. 

Fold in dark chocolate chips. 

Transfer batter to the greased loaf pan. Top with mini chocolate chips.

Bake for 1 hour to 1 hour 10 minutes, until a toothpick comes out clean when inserted in the middle. Let cool in the pan completely before removing from the pan.