Preheat your oven to 350 and prepare a muffin tin with 12 muffin liners.
Use a food processor or a blender if you don't have one to combine the nut butter, mashed banana, spinach, baking soda, vanilla, eggs, salt, honey (or maple syrup), and cinnamon, if using, and mix until smooth.
Mix in one of the optional add-ins. I prefer to add a few on the top after the liners have been filled as well.
Divide the batter among the 12 (give or take) muffin cups. Approximately 1/4 cup each of the batter in each.
Bake 20 to 25 until the muffins have risen, and the center of the muffin poked with a toothpick comes out clean. Allow to cool for at least 30 minutes before removing from the pan. These muffins should be stored them in the fridge once cooled. You can freeze them for up to 3 months in an airtight container, too.