In a large skillet, cook breakfast sausage over medium heat until browned and crumbled. Drain and set aside.
Whisk together eggs, milk, salt, and pepper. Melt butter in the same skillet and scramble eggs until just set but still slightly creamy.
Warm tortillas slightly to make them more pliable. Lay one tortilla flat and add a portion of scrambled eggs down the center.
Top with sausage, black beans, cheese, bell peppers, and a spoonful of salsa if using. Don't overfill!
Fold in the sides of the tortilla, then roll tightly from bottom to top. Wrap each burrito individually in foil, then place in a freezer bag.
Freeze for up to 3 months. To reheat: Remove foil, wrap in a damp paper towel, and microwave for 2-3 minutes, flipping halfway through. Or bake in foil at 350°F for 25-30 minutes.