Preheat oven to 350F degrees and line two 12-cup muffin pans with 18 paper liners or spray with non-stick cooking spray. If you only have one muffin tin, that's fine, you can bake one batch then the remainder of the batter afterwards (it's fine for the batter to sit while one set is baking).
Whisk together the dry ingredients in a large bowl: Flours, sugar, baking powder, baking soda, and salt. Set aside.
In a blender, place the oil, milk and spinach. Blend on high for about 30 seconds or until completely pureed. Add the banana and vanilla; blend on low just to mix.
Pour the pureed mixture into the combined dry ingredients and fold together until completely combined.
The batter will be thin, like pancake batter. Let it sit for about 5 minutes to thicken slightly.
Pour batter into muffin cups, filling 2/3 to 3/4 full. Using your fingers to pinch a bit of sugar, sprinkle over the tops of the muffins lightly with the sugar to thinly but completely cover the tops. This is optional but will provide a delicate crispness to the tops of the muffins. Don't use brown sugar or anything other than regular granulated sugar for this or the tops of the mufffins could get a bit burnt and won't look shiny and appetizing.
Bake 20 minutes on the middle rack of the oven. Once the tops of the muffins look bright and not too shiny, turn off the oven and open the oven door completely. Slide out the middle rack of the oven with the muffin pan still on the rack, so the pan is still inside the oven but barely. Let the muffins sit like this for 10-15 minutes; this prevents the muffins from sinking but also prevents them from being overcooked and dry.
Remove the pan from the oven and cool completely on a cooling rack. Serve immediately and place any extras in the refrigerator in an airtight container for up to 2 days.