Preheat the oven to 350 degrees.
Spray a 24-cup mini muffin pan with non-stick cooking spray and set aside.
In a large mixing bowl, crack eggs and add milk. Whisk together. Add pepper flakes if you like a little spice. Whisk again to combine. Set aside.
Fill each muffin cup about halfway with ground chicken and spinach, evenly distributing the mixture among the cups.
Pour the egg mixture into each muffin cup, leaving a little bit of room at the top for the muffins to rise.
Bake for 15 minutes, or until the egg has set, and the edges of the cups are browned. The middle should be firm to the touch.
Remove from the oven and let them cool for about 5 minutes before serving.
If the cups don't slide out easily, use a spoon to pop them out. Serve immediately or refrigerate for up to three days.