Bring a large pot of water to a boil. Add the macaroni and cook until al dente according to the package directions. Drain and set aside.
In a medium pot over medium heat, melt the butter. Whisk in the flour, ground mustard, garlic powder, salt and pepper. Slowly add the cream and milk, whisking until combined. Heat the mixture until it bubbles, 3 to 4 minutes. Reduce the heat to low and add the Cheddar, Parmesan, processed cheese and sour cream. Stir until melted. Add the macaroni to the cheese mixture and toss to combine. Bring the mixture back to a light simmer to heat everything through and thicken. Serve warm.