Preheat oven to 425°F; bake chicken on a rimmed baking sheet for 20 minutes.
Mix 2 tablespoons hot sauce, melted butter, and 1 teaspoon ranch seasoning in a bowl.
Cut chicken into ½-inch pieces and coat in hot sauce mixture.
In a 12-inch broiler-safe skillet, bring 1¾ cups water and 1 cup milk to a boil over medium-high heat.
Stir in macaroni, reduce heat to medium-low, cover, and cook for 5 minutes, stirring once.
Stir in American cheese, 1 tablespoon hot sauce, and 1 teaspoon ranch seasoning until cheese melts, about 2 minutes.
Remove from heat, stir in cheddar cheese until melted, about 2 minutes.
Heat broiler; evenly distribute chicken over macaroni and broil for 3 minutes.
Drizzle with 2 tablespoons hot sauce, sprinkle with scallions, and serve immediately.