01 - Bring a large pot of salted water to a rolling boil. Cook spaghetti according to package directions until al dente. Reserve 1/2 cup of the starchy pasta water before draining.
02 - Melt butter in a large skillet over medium heat. Add minced garlic and sauté for 1 to 2 minutes until fragrant and lightly golden, being careful not to burn.
03 - Pour heavy cream into the skillet and bring to a gentle simmer. Gradually whisk in Parmesan and mozzarella cheeses until fully melted and the sauce is smooth and velvety.
04 - Add salt, black pepper, and red pepper flakes if using. Stir to incorporate evenly throughout the sauce.
05 - Add cooked spaghetti to the skillet. Toss thoroughly to coat noodles in sauce, adding reserved pasta water a tablespoon at a time until desired consistency is reached.
06 - Remove from heat immediately. Sprinkle with fresh chopped parsley and additional Parmesan cheese. Serve hot while the cheese is melted and creamy.