Preheat your oven to 350°F (175°C).

In a high-powered blender, combine the milk, ripe bananas, and baby spinach. Blend until mostly smooth with a few lumps remaining. Transfer this mixture into a large mixing bowl.

Add the egg to the mixture and lightly whisk to combine. Then stir in the sugar, vanilla extract, oil, and Greek yogurt until well incorporated.

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

Gradually add the dry ingredient mixture to the wet ingredients, gently folding it in until fully combined.

Stir in half of the dark chocolate chips into the batter. Save the remaining chocolate chips for topping.

Line a 12-cup muffin pan with muffin liners. Spoon the batter evenly into each cup, filling about two-thirds full. Sprinkle additional chocolate chips on top if desired.

Bake the muffins in the preheated oven for 18-22 minutes. Check doneness by inserting a toothpick into the center of a muffin; it should come out clean or with a few moist crumbs.

Remove the muffins from the oven and let them cool in the pan for a few minutes before transferring them to a wire rack to cool completely.