Prepare the eggs:
In a medium bowl, whisk together the eggs, milk (if using), a pinch of salt, and pepper until well combined. Heat the butter or oil in a nonstick skillet over medium heat. Add the egg mixture and cook, stirring gently with a spatula, until the eggs are softly scrambled. Remove from heat and set aside.

Season the chicken:
In the same skillet, toss the shredded chicken with cumin, smoked paprika, and a pinch of salt. Cook over medium heat for 2-3 minutes, stirring to combine and warm the chicken through. Set aside.

Warm the tortillas:
Heat each tortilla in a dry skillet or in the microwave for about 10 seconds to make them soft and pliable.

Assemble the burritos:
Lay a tortilla flat on a clean surface. Spread a tablespoon of sour cream or salsa (if using) in the center. Add a portion of scrambled eggs, seasoned chicken, diced tomatoes, bell peppers, green onions, shredded cheese, and avocado slices. Be careful not to overfill!

Wrap it up:
Fold in the sides of the tortilla, then roll it up tightly from the bottom to form a burrito. Repeat with the remaining tortillas and fillings.

Serve or store:
Serve the burritos immediately with extra salsa or hot sauce on the side. For a crispy finish, you can lightly toast the burritos in a skillet over medium heat for 1-2 minutes per side. If meal prepping, wrap each burrito tightly in foil or parchment paper and store in the fridge for up to 3 days.