Preheat your oven to 375°F (190°C) and generously grease a 12-cup mini muffin tin.
In a large bowl, whisk the eggs and milk together until fully combined and frothy.
Stir in the cheddar cheese, spinach, zucchini, garlic powder, and salt (if using) until evenly distributed.
Pour the egg mixture evenly into the prepared muffin cups, filling each about 3/4 full.
Bake for 12-15 minutes, or until the bites are puffed up and set in the center.
Let cool in the pan for 5 minutes, then run a knife around the edges and transfer to a wire rack.