01 - Bring a large pot of salted water to a boil. Cook spaghetti according to package directions until al dente. Reserve 1/2 cup pasta water before draining.
02 - Melt butter in a large skillet over medium heat. Add minced garlic and sauté 1–2 minutes until fragrant, being careful not to brown.
03 - Pour heavy cream into skillet and bring to gentle simmer. Stir in Parmesan and mozzarella cheeses until completely melted and smooth.
04 - Add salt, black pepper, and red pepper flakes if using. Stir well to incorporate seasonings.
05 - Add cooked spaghetti to skillet. Toss to coat noodles, 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. Serve immediately.