Bring a large pot of salted water to a boil. Add the spaghetti and cook until al dente, about 8–10 minutes per the package. Reserve ½ cup of the hot pasta water, then drain the spaghetti.

Heat a large skillet over medium. Add the butter and olive oil and let the butter melt.

Add the minced garlic and sauté 1–2 minutes until fragrant and lightly golden. Watch it closely so it does not burn.

Reduce the heat to low. Add the grated Parmesan and shredded mozzarella to the skillet and stir until they start to melt.

Gradually pour in the reserved pasta water, stirring, until the sauce becomes smooth and silky. Use more or less water to reach your preferred consistency.

Add the drained spaghetti to the skillet and toss until every strand is evenly coated. Season with salt and pepper to taste.

Plate the pasta and finish with chopped parsley and extra Parmesan on top.