Preheat the oven to 375 degrees F. Spray muffin tins with cooking spray and set aside.
In a medium-sized bowl, whisk together the flour, cocoa powder, salt, and baking soda. Set aside.
In the bowl of a stand mixer with the paddle attachment, beat the bananas until they are well mashed. Add the eggs, vanilla extract, olive oil, and milk. Mix until everything is combined. Add the sugar and mix until combined again.
In a couple of batches, add the dry ingredients to the wet. Scrape down the sides of the mixing bowl as needed. Fold in the chocolate chips and chopped walnuts.
Fill the muffin tins up almost halfway with muffin batter. Top with some more chocolate chips and chopped walnuts. Bake 15-20 minutes, or until a toothpick or knife, when inserted in the center, comes out clean.
Cool for 5 minutes in the muffin tin before removing the muffins to a wire rack to finish cooling completely.