Preheat your oven to 350°F (175°C).
Cook the elbow macaroni according to the package directions until al dente.
While the pasta cooks, melt the butter in a large saucepan over medium heat.
Add flour to the melted butter and whisk constantly for about 1 minute.
Slowly pour in the milk while continuously whisking, until the mixture is smooth.
Stir in the garlic powder, salt, and pepper, adjusting seasoning to your taste.
Reduce heat to low and add the shredded cheddar and mozzarella cheese, stirring until melted.
Add the shredded chicken and Buffalo sauce to the cheese mixture, stirring well to combine.
Combine the cooked pasta with the cheese and chicken mixture, stirring until well blended.
Transfer the mixture to a greased baking dish and spread evenly.
Bake in the preheated oven for 20–25 minutes, or until bubbly and golden on top.
Remove from oven, allow to cool slightly, and finish with green onion garnish before serving.