In a separate large pot, melt the six tablespoons of butter over medium heat. Add the all purpose flour and whisk to create a roux. Cook the roux for 2 minutes, whisking constantly. Add your spices and whisk them in as well.
6 tablespoon butter, ½ cup all purpose flour, ½ teaspoon dry mustard powder, 1 teaspoon garlic powder, 2 teaspoon kosher salt, 1 teaspoon onion powder, 2 teaspoon freshly ground black pepper
Add the milk and heavy cream, about ¼ cup at a time, whisking well with each addition. After about two cups of liquid has been added, pour in the rest of the milk and heavy cream and whisk well to combine.
3 cups whole milk, 1 cup heavy cream
Cook for 5-7 minutes, whisking frequently so the bottom doesn't scorch. Once the bechamel sauce is thick enough to coat the back of a spoon and stay separated when you run a finger down that spoon, it's ready for cheese.
Turn the temperature to low and add a handful of the cheese at a time, whisking in each handful before adding the next. Once the cheeses are fully incorporated and melted, turn the heat off and stir in the sour cream.
1 cup sharp cheddar cheese, 1 cup mozzarella cheese, 2 cup gouda cheese, ½ cup full-fat sour cream
Once your pasta is just under al dente, drain it and stir the macaroni into the cheese sauce thoroughly. Taste the cheese sauce and season with salt and pepper as needed.