Prep: preheat the oven to 350° F. Spray or line a mini muffin tin.
Dry Ingredients: in a medium bowl, whisk together the flour, cinnamon, baking soda, baking powder, and salt.
Wet Ingredients: in a blender, add the spinach, milk, applesauce, oil/butter, maple syrup, egg, and vanilla extract. Blend until completely smooth and there are no chunks of spinach - roughly 45-60 seconds.
Combine: pour the spinach mixture into the dry ingredients and stir until just combined. Do not overmix.
Fill Muffin Tin: Scoop the batter into the muffin tin. For mini muffins - use roughly 1 tablespoon per cup. For regular muffins - use roughly 1/4 cup per cup.
Bake: Place the muffin tin into the oven, and bake for 12-14 minutes for mini muffins and 18-20 for regular size muffins or until just golden brown.
Eat: Let cool for 5-10 minutes, and serve.