Prep: Preheat oven to 375 degrees F. Heavily spray a muffin tin with cooking spray. To make getting the egg cups out easier, I have found that a silicone muffin tin or silicone muffin inserts help tremendously with the eggs not getting stuck to the pan.
Whisk: In a medium bowl, whisk together the eggs, milk, salt and pepper until well combined.
Add Ingredients: Evenly distribute the add-in ingredients into each muffin cup, until they are 1/2-2/3 the way full.
Add Egg Mixture: Evenly pour the egg mixture into each muffin cup until the each cup is roughly 2/3 full.
Bake: Place the muffin tin into the oven and bake for 20 minutes or until eggs have firmed and tops are golden brown and bubbly.