Preheat the oven to 350F (180C) degrees and line 2 muffin trays with muffin papers. You'll end up with about 14-16 cupcakes
In a large bowl whisk together the flour, cocoa, baking soda, baking powder and salt. Set aside.
In a separate large bowl whisk together the oil, brown sugar, granulated sugar, eggs, vanilla extract and milk. Then whisk in the mashed banana
Make a well in the middle of the dry ingredients, then pour the wet ingredients into the dry ingredients. Carefully whisk together until you no longer see lumps or streaks of dry ingredients. There will still be lumps of banana.
Spoon the batter into the lined muffin tin. Fill the muffin papers about 2/3 to 3/4 full. You should end up with around 14-16 cupcakes in total.
Bake 1 muffin tin at a time on the middle rack of the preheated oven. They'll need to bake for about 16-18 minutes, or until an inserted toothpick comes out clean or with a few damp crumbs.
Cool cupcakes in the muffin tin for at least 10 minutes, then carefully transfer to a wire rack to continue cooling.