Preheat the oven to 350 degrees F. Grease a mini muffin pan with cooking spray.
Place the spinach, applesauce, sugar, oil, vanilla extract and egg in a food processor and puree. Combine the flour, baking powder, baking soda and salt in a separate bowl. Pour the spinach puree in a large mixing bowl. Slowly mix the dry ingredients into the wet ingredients until combined.
Scoop the batter into the greased mini muffin pan, filling each cup two-thirds of the way. Bake for 12 minutes. Let cool completely, then serve or freeze.
To freeze for later, place cooled muffins on a parchment-lined baking sheet. Freeze until solid, about 1 hour. Transfer to a freezer bag and store for up to 3 months.