Preheat the oven to 350°F. Spray a 8 x 12-inch baking dish with cooking spray. Set aside.
Cook the macaroni in salted water per the package directions until al dente.
While the macaroni cooks, in a separate pan over medium-high heat cook together 6 Tbsp of butter, 2 Tbsp reserved bacon drippings and onion.
Cook for 3 minutes until the onion has softened. Lower the heat to medium and add the flour, garlic salt, mustard and black pepper. Stir until the flour has absorbed the butter. Cook stirring constantly for 1 minute.
Gradually whisk in the milk. Bring to a boil then immediately lower the temperature to low heat. Allow to bubble gently for 5 minutes.
Remove the sauce from the heat and add 3 cups of cheddar cheese and crumbled bacon. Reserve 2 Tbsp crumbled bacon for garnishing. Stir until the cheese has melted.
Drain the macaroni well, then mix together with the sauce and sliced hot dogs. Mix well and pour into the baking dish.
Top with the remaining 1 cup of cheese. Dot with 1 Tbsp butter.
Bake for 30-35 minutes until golden and bubbly.
Garnish with bacon crumbles and sliced green onion. Serve immediately.