Preheat the oven to 350 degrees F. Line a 12-count muffin tin with paper liners and set aside.
Add the spinach to the bottom of the blender pitcher, then add all the ingredients except for the baking powder on top of the spinach. (This helps make sure the spinach actually blends!) Blend on high for 30-45 seconds, or until the batter becomes green and there are no pieces of spinach.
Once fully blended, add the baking powder and blend for another 5 seconds – just long enough to incorporate it without overmixing.
Divide the batter evenly between the muffin wells and bake for 15-18 minutes (16 minutes was perfect for my oven), or until the muffins have fully risen and are no longer gooey in the center.
Allow muffins to cool fully to ensure they can be removed cleanly from the paper liners.