Preheat the oven to 350°F (175°C).
Slice each croissant horizontally without cutting all the way through, creating a pocket for the filling.
In a medium bowl, whisk together the eggs, whole milk, salt, and black pepper.
Heat the unsalted butter in a non-stick skillet over medium heat until melted.
Pour the egg mixture into the skillet and cook, stirring gently, until the eggs are softly scrambled, about 3-4 minutes. Remove from heat.
In a separate skillet, cook the breakfast sausage links over medium heat until browned and cooked through, about 8 minutes. Drain on paper towels and slice each link into 3 pieces.
Open each croissant and layer approximately 1/6 of the scrambled eggs inside, followed by 1/6 of the sliced sausage pieces and 2 tablespoons of shredded cheddar cheese.
Close the croissants and wrap each individually in aluminum foil.
Place the wrapped croissants on a baking sheet and freeze for at least 2 hours or until solid.
To reheat, preheat the oven to 350°F (175°C). Remove the foil and wrap the croissant in a new piece of foil. Bake for 15-20 minutes or until heated through and the cheese is melted.