01 - Bring a large pot of salted water to a boil. Cook the spaghetti according to package instructions until al dente. Reserve 1/2 cup pasta water, then drain the pasta.
02 - While the 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 in the heavy cream and bring to a gentle simmer. Stir in the Parmesan and mozzarella cheese until melted and smooth.
04 - Season with salt, black pepper, and red pepper flakes if using.
05 - Add the cooked spaghetti to the skillet. Toss to coat, adding reserved pasta water a little at a time until the sauce reaches your desired creaminess.
06 - Remove from heat. Sprinkle with chopped parsley and extra Parmesan before serving.