Combine macaroni, milk, water, garlic salt, and salt in a Dutch oven over medium-high heat; bring to a boil, stirring occasionally. Reduce heat to a simmer; cook, stirring frequently until macaroni is soft and most of the liquid has been absorbed, 7 to 10 minutes.

Off heat, stir in margarine and black pepper until melted. Stir in Cheddar cheese and 1/4 cup Parmesan cheese until melted.

Top macaroni and cheese with bread crumbs, remaining 1/4 cup Parmesan cheese, and bacon bits.

Set an oven rack about 6 inches from the heat source and preheat the oven’s broiler.

Broil until topping is lightly browned and crispy, 1 to 3 minutes. Top with chives and serve immediately.