Preheat oven to 350°F.
Combine milk, bananas and spinach in a high powered blender and blend until mostly smooth. A few lumps is ok.
Transfer "smoothie" to a medium mixing bowl.
Add egg and stir to lightly whisk.
Add sugar, vanilla, oil and Greek yogurt and stir to combine.
In a small bowl, combine flour, baking soda, sea salt and cocoa powder and stir to combine.
Slowly add dry ingredients to wet ingredients and stir to fold in. Stir in the chocolate chips, saving some or adding extra in for topping.
Line a 12 muffin pan with muffin liners and spoon batter into each.
Add chocolate chips to the tops of each muffin.
Bake for about 18-22 minutes or until you can put a toothpick in the center and it comes up clean.
Cool for several minutes before serving.
Store in an airtight container for up to 3-5 days.