Shred white cheddar cheese or cut it into small pieces.
In a pot, bring water to boil and add shells. Cook according to package instructions. Once pasta is cooked, drain water and set aside.
Heat a separate pot. Add butter, and once butter is melted (do not brown butter) add flour. Use a whisk to combine butter and flour.
Add heavy cream, garlic powder, and salt to the pot. Bring to boil while whisking and stirring constantly (See Note 1). Once mixture reaches boiling point, continue stirring using whisk and let it boil for another 1-2 minutes or until mixture is smooth.
Lower heat to medium low and add shredded white cheddar and slices of American cheese. Stir until cheese is melted and the sauce is smooth. Remove from heat.
Add pasta to the cheese mixture and toss to coat evenly.
Optional: If sauce is too thick, add a tbsp of milk, heat it on low heat and stir. Repeat until you get it to the consistency you desire. If sauce is too thin, heat on low heat and continue to stir and cook until sauce thickens to your desired consistency.
Sprinkle with fresh parsley if desire. Serve and enjoy!
Note 1: Whisk is used to dissolve flour in the mixture