Preheat the oven to 350 degrees F.
In a 12-inch deep skillet, melt the butter over medium heat; add the flour, whisking to combine.
Cook the roux (the flour and butter mixture) for 1-2 minutes until deep golden brown in color; remove from the heat.
Slowly add the milk, whisking until the mixture is smooth. Once the sauce is nicely smooth, return the skillet to the burner, still over medium heat.
Return the skillet to medium heat, then add the garlic powder, seasoned salt, Parmesan, and cubes of Velveeta cheese.
Cook, stirring often, until the sauce is smooth and the Velveeta is fully melted. Add all but ½ cup of the shredded cheddar cheese to the sauce, a handful at a time, stirring between each addition until the cheese is melted and the sauce is smooth.
Fold in the cooked spaghetti noodles. then transfer the spaghetti mac and cheese to a greased 9x13 pan.
Sprinkle the remaining ½ cup shredded cheddar over the top of the spaghetti mac and cheese, then bake, uncovered, for 30-35 minutes, or until hot and bubbly throughout (165 degrees F. at the center).
Remove the spaghetti mac and cheese from the oven and let rest for 5 minutes before serving.
Bake, uncovered, for 35-40 minutes, or until hot and bubbly throughout. (160-165 degrees F. at the center of the casserole is a hot and safe internal temperature.)
