Preheat the oven to 400°F (200°C). Line a 12-cup muffin pan with paper liners or spray it with non-stick spray and set aside.
In a medium bowl whisk together the all-purpose flour, whole wheat flour, baking powder, baking soda, and salt.
Stir the dark chocolate chips into the dry ingredients to help prevent them from sinking.
In a separate bowl combine the mashed bananas, vanilla extract, Greek yogurt, melted and cooled coconut oil, eggs, and brown sugar; mix until smooth.
Add the wet mixture to the dry ingredients and gently stir until just combined; do not overmix.
Spoon or scoop the batter into the prepared muffin cups, filling each about 3/4 full.
Bake at 400°F for 6 minutes, then without opening the oven lower the temperature to 350°F and bake for an additional 12–14 minutes, until a toothpick inserted into a muffin comes out clean.
Let the muffins cool in the pan for a few minutes, then transfer to a wire rack to cool to room temperature before serving.