Preheat oven to 180°C (350°F). Line a 12-cup muffin tin with paper liners or lightly grease.
In a large bowl, whisk together flours, cocoa powder, baking powder, baking soda, salt, and sugar.
In a small bowl, combine the milk and vinegar. Let sit for a few minutes while you prepare the other ingredients — this creates a simple vegan buttermilk.
In a separate bowl, mash the banana, then add the oil, vanilla, and buttermilk mixture. Whisk until smooth.
Pour wet ingredients into the dry, mix gently until just combined — don’t overmix.
Fold in the squeezed zucchini and chocolate chips.
Divide batter evenly into muffin cups.
Bake for 22–25 minutes, until a skewer inserted in the centre comes out clean or with moist crumbs.
Cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.