Prepare a muffin pan, by brushing the muffin tray with melted butter or by spraying with cooking spray.
In a bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set this aside.
In a large mixing bowl, add the eggs, chopped spinach, melted butter, and whisk gently to combine.
Add the corn, feta cheese, and cheddar cheese and mix gently.
Add the milk, followed by the flour mixture slowly, in two additions, gently whisking to combine.
Transfer the batter to the prepared muffin pan, and bake for 20 minutes until the muffins are done. The muffins should be a light golden brown when done. Check with a toothpick to see if the muffins are done, before taking them out.
Let the muffins cool before you serve them or refrigerate or freeze for later.
Notes
You could also replace fresh spinach with frozen spinach cubes. Thaw before using.