Preheat the oven to 350 degrees F. Grease a 9-by-5-inch nonstick loaf pan with baking spray or vegetable oil and set aside.
Combine the bananas, oil, sugar, vanilla and milk in the bowl of a stand mixer fitted with the paddle attachment. Beat until smooth. Whisk together the flour, cocoa powder, baking powder, baking soda and salt in a medium bowl. Add the dry ingredients to the wet and mix until smooth. (Alternatively, whisk together the wet ingredients in a large bowl, then stir in the dry ingredients with a wooden spoon.) Fold in the chocolate chips, then pour the batter into the prepared pan.
Bake until the cake has risen, is slightly cracked and a toothpick inserted in the center comes out clean, 45 to 55 minutes.
Let cool in the pan for 10 minutes, and then run a knife around the edges to loosen and remove from the pan. Transfer to a wire rack to cool completely.
You could substitute coconut oil but keep in mind that it will add a slight coconut flavor to the bread.
Note that not all chocolate chips are vegan, so be sure to check the label for the ingredients.