Preheat oven to 350°F and line two 12-cup muffin pans with paper liners.
Whisk together dry ingredients in a large bowl: flours, sugar, baking powder, baking soda, cinnamon, and salt. Set aside.
In a blender, place oil, milk, and spinach. Blend on high for about 30 seconds or until completely puréed. Add banana and vanilla; blend on low just to mix.
Pour puréed mixture into dry mixture and fold together with a rubber spatula until completely combined.
Fill muffin cups about ⅔ full and bake 18-20 minutes or until a toothpick inserted into the center comes out clean.
Serve and enjoy!