Bring a large pot of salted water to a boil. Add 8 ounces of elbow macaroni and cook according to package instructions until al dente, about 7-8 minutes. Drain the pasta and set it aside.
In a large mixing bowl, combine 2 cups of shredded cooked chicken with 1 cup BBQ sauce and 1 cup ranch dressing. Stir until the chicken is well coated in the sauces.
Add the cooked macaroni to the chicken mixture. Sprinkle in 2 cups shredded sharp cheddar cheese, 1 cup shredded mozzarella cheese, 1 teaspoon garlic powder, 1/2 teaspoon black pepper, and 1/2 teaspoon salt. Toss everything together until the cheese starts to melt and the pasta is evenly coated.
Preheat your oven to 350°F (175°C). Lightly grease your 9x13-inch baking dish. Pour the mac and cheese mixture into the dish, spreading it out evenly. Sprinkle 1/2 cup chopped green onions on top.
Heat 1 tablespoon olive oil in a small skillet over medium heat. Add 1/2 cup breadcrumbs and toast them, stirring frequently, until golden brown and fragrant, about 3-5 minutes. Remove from heat.
Sprinkle the toasted breadcrumbs evenly over the top of the mac and cheese. Place the baking dish in the oven and bake for 20-25 minutes, or until the top is golden and bubbly.
Remove from the oven and let it cool for a few minutes before serving. This bake is perfect on its own or paired with a fresh salad or steamed veggies.