Preheat oven to 350 degrees and line a standard 12-cup muffin tin with unbleached parchment liners.
In a large bowl, whisk together the flour, baking powder, baking soda and salt.
In a blender add egg, maple syrup, milk, oil, vanilla and spinach. Process until pureed.
Add spinach mixture to the flour mixture and stir until just combined. Fold in the mashed banana.
Scoop batter into prepared muffin tin, filling cups about ¾ full. I like to do this with a large ice cream scooper. Bake about 25 minutes, or until toothpick inserted into the center comes out clean.
Store at room temperature up to 3 days. These freeze nicely!