Preheat the oven to 350°F. Spray a 12 x 8 inch baking dish with cooking spray. Set aside.
Cook the macaroni in salted water per the package directions until al dente. Drain well.
Meanwhile, in a separate pan over medium-high heat add 6 Tbsp of butter to reserved 2 Tbsp bacon drippings. Add 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 to thicken.
Remove sauce from heat. Add 3 cups cheddar cheese and crumbled bacon. Reserve 2 Tbsp crumbled bacon for garnishing. Stir until the cheese has melted.
Pour sauce over macaroni. Add hot dogs. Mix until macaroni is evenly coated and hot dogs are evenly distributed. 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.