Preheat oven to 350 degrees F (175 degrees C). Lightly grease 12 cup muffin cups.
Mix the flour, baking powder, baking soda, and salt together in a mixing bowl.
Stir the melted butter, egg, milk, spinach, and Cheddar cheese together in a large mixing bowl until evenly blended. Slowly stir in the flour mixture to form a batter. Spoon about 2 tablespoons into each muffin cup.
Bake in preheated oven until a toothpick inserted into the center of a muffin comes out clean, about 35 minutes.