In a skillet over medium heat, melt 4 Tbsp. butter. Cook fettuccine pasta according to package instructions in a separate pot.
Once the butter melts, add in 1 Tbsp. minced garlic and cook for 2-3 minutes while stirring.
Slowly and gradually add in 2 cups heavy cream. Stir, stir, stir! This is very important - you don't want the cream to curdle. Stir while you pour the heavy cream into the skillet and add it over time, not all at once.
After adding the cream, slowly and gradually add in ½ cup Parmesan. Keep stirring while you add it!
Add in 2 to 3 slices of white American cheese and stir well.
Sprinkle with freshly cracked pepper and stir to combine.
Finally, add in your cooked pasta and mix together. Top with additional Parmesan cheese and parsley to taste. I like to also add in chicken, steak, or shrimp. This is a great dish for using up leftovers!
Make sure to gradually add your heavy cream and Parmesan cheese! You don't want the cream to curdle or the cheese to stick to the bottom of the pan.