Preheat oven to 350°F/175°C and prepare a muffin pan with liners.
In a large bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt.
Warm the milk slightly, to about room temperature. Add the spinach, melted coconut oil, and milk to a blender and blend until completely pureed. Add the mashed banana, honey, and vanilla to the spinach mixture in the blender and blend at a low speed, just to combine. Pour spinach mixture into the large bowl with dry ingredients and gently stir until combined.
Fill muffin cups about 3/4 full and bake immediately for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Remove the muffins from the pan and let cool on a cooling rack.
Notes
I get about 13 muffins worth of batter, so you may (or may not) want to consider preparing a second pan.