Preheat your oven to 375°F and grease a 9x13-inch baking dish.
Prepare the cornbread batter according to the package instructions (combine cornbread mix, 1/3 cup milk, and 1 egg). Pour the batter into the baking dish and bake for 8-10 minutes, or until just set but not fully cooked.
While the cornbread is baking, heat a skillet over medium heat and cook the sausage until browned and crumbly. Drain any excess grease and set aside.
In a mixing bowl, whisk together the remaining 4 eggs, 1 cup milk, salt, pepper, and garlic powder until well combined.
Remove the cornbread from the oven and layer the cooked sausage evenly over the top. Sprinkle the shredded cheddar and mozzarella cheeses over the sausage.
Pour the egg mixture evenly over the entire casserole. Use a spatula to gently press down to ensure the liquid saturates the cornbread and toppings.
Return the dish to the oven and bake for 30-35 minutes, or until the eggs are fully set and the cheese is melted and bubbly.
Let cool for 5 minutes before slicing and serving.