Cook the pasta in boiling salted water according to the package instructions. Drain and set aside.
Cut the bacon into bite-size pieces and cook in a large pot over medium heat until crispy. Remove from the pot and set aside.
Make a roux (thickening paste) by mixing the cornstarch and salt with the rendered bacon grease. Cook and stir over medium heat for about 5 minutes.
Pour in the milk and bring to a boil. Reduce the heat, simmer and stir until slightly thickened. About 10 minutes.
Turn the heat to low and stir in the sour cream and shredded cheeses until melted and smooth.
Stir the drained pasta into the cheese sauce until evenly coated. Stir in the cooked bacon. I like to reserve some of the cooked bacon to use as a topping. Serve warm, garnish with chopped fresh parsley, if desired.
Cover leftovers and store in the refrigerator for 3-5 days.