Preheat the oven: Preheat your oven to 350°F (175°C). Line a 12-cup muffin pan with muffin liners or lightly grease it.

Blend the wet ingredients: In a high-powered blender, combine the milk, ripe bananas, and baby spinach. Blend until the mixture is mostly smooth (a few small lumps are okay). Pour the blended mixture into a large mixing bowl.

Add the remaining wet ingredients: Lightly whisk the egg into the banana-spinach mixture. Stir in the sugar, vanilla extract, oil, and Greek yogurt until well combined.

Prepare the dry ingredients: In a separate small bowl, whisk together the white whole wheat flour, baking soda, cocoa powder, and sea salt.

Combine the wet and dry ingredients: Gradually fold the dry ingredients into the wet mixture, stirring gently to combine. Be careful not to overmix, as this can result in dense muffins. Stir in the dark chocolate chips, reserving a few to sprinkle on top of the muffins if desired.

Fill the muffin pan: Spoon the batter evenly into the lined muffin cups. Add extra chocolate chips to the tops of the muffins for a little more indulgence.

Bake: Bake for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean or with just a few moist crumbs.

Cool and enjoy: Allow the muffins to cool in the pan for several minutes before transferring them to a wire rack to cool completely. Enjoy warm or at room temperature.