01 - Bring a large pot of salted water to a boil. Cook spaghetti according to package directions until al dente. Reserve 1/2 cup of pasta water, then drain.
02 - While pasta cooks, melt butter in a large skillet over medium heat. Add minced garlic and sauté for 1–2 minutes until fragrant but not browned.
03 - Pour heavy cream into skillet and bring to a gentle simmer. Stir in Parmesan and mozzarella cheeses until melted and smooth.
04 - Add salt, black pepper, and red pepper flakes if using. Adjust seasoning to taste.
05 - Add cooked spaghetti to the skillet. Toss to coat, adding reserved pasta water a little at a time until sauce reaches desired consistency.
06 - Remove from heat. Sprinkle with chopped parsley and additional Parmesan cheese before serving.