Preheat the oven to 350 degrees F (175 degrees C). Coat 10 muffin cups with cooking spray.

Combine spinach, egg whites, Cheddar cheese, hot sauce, salt, and black pepper in a bowl. Ladle mixture into the prepared muffin cups, filling each cup 3/4 full.

Bake in the preheated oven until a knife inserted into centers of muffins comes out clean, 20 to 25 minutes. Serve warm or cooled.