Bring a large pot of well-salted water to a boil. Cook the elbow macaroni about 1 minute less than package directions for al dente, then drain and set aside.
In a medium saucepan over medium heat, melt 3 tablespoons of butter. Whisk in the flour and cook for about 90 seconds, until pale and sandy-smelling.
Slowly whisk in the milk in a steady stream, whisking constantly, until the mixture thickens enough to coat the back of a spoon, about 4 minutes.
Reduce heat to low. Add the shredded cheddar and Monterey Jack in two additions, stirring until fully melted before adding more. Stir in the garlic powder and smoked paprika.
Remove the saucepan from the heat completely. In a small bowl, melt the remaining 3 tablespoons of butter and whisk it into the Frank's RedHot. Pour into the cheese sauce and stir until fully combined. Season with salt and pepper to taste.
Fold the drained macaroni into the buffalo cheese sauce until evenly coated. Transfer to a broiler-safe baking dish.
Toss the panko with the 1 tablespoon of melted butter, then sprinkle evenly over the top. Broil for 2–3 minutes, watching closely, until the panko is deep golden brown.
Top with crumbled blue cheese and chopped green onion if using. Serve immediately.