Preheat the oven to 350 degrees. Spray two muffin tins with cooking spray (my favorite muffin tins here, affiliate link).
To a blender, add the bananas, eggs, buttermilk, sugar, spinach and vanilla. Blend on high for 30 seconds to a minute until the mixture is smooth and creamy. With the blender running, drizzle in the melted coconut oil and blend together until the ingredients are completely blended together.
To a large bowl, add the whole wheat flour, baking powder, baking soda and salt. Whisk together.
Pour the spinach banana liquid into the flour mixture and gently combine with a large spoon. Stir until only a few streaks of flour mixture remain (be careful not to over-mix).
Fill each muffin tin about half full (about 1/4 cup batter per muffin tin) until all 24 tins are filled.
Bake for 16-18 minutes until muffins are baked all the way through. Stick a toothpick in the middle of a muffin. If it comes out clean, the muffins are ready. If a little wet batter is stuck to the toothpick, the muffins need a few minutes longer.
Let the muffins cool for 5-10 minutes and then remove from the tins to a cooling rack. Enjoy!