Cook the Pasta: Bring a large pot of well-salted water to a rolling boil. Add the spaghetti and cook according to the package directions until it's al dente (firm to the bite).

Reserve Pasta Water: Just before draining, carefully scoop out and reserve about 300ml of the starchy pasta water. This is a crucial step! Then, drain the pasta in a colander.

Sauté the Garlic: While the pasta is cooking, melt the butter in a large, deep frying pan over a medium-low heat. Add the minced garlic and cook gently for 1-2 minutes, stirring frequently, until it's fragrant and softened but has not taken on any colour. You don't want it to brown, as it will become bitter.

Start the Sauce: Pour the double cream into the pan with the garlic butter. Stir to combine and bring the mixture to a very gentle simmer. Let it bubble softly for a minute to thicken slightly.

Add the Cheese: Reduce the heat to its lowest setting. Add the freshly grated Parmesan cheese to the pan, a little at a time, whisking or stirring constantly until it has completely melted into a smooth sauce. Taking the pan off the heat entirely can help prevent the cheese from clumping.

Season: Season the sauce with salt and freshly ground black pepper. Add the red pepper flakes now, if you're using them.

Combine Pasta and Sauce: Add the drained pasta directly into the frying pan with the sauce. Use tongs to toss everything together thoroughly.

Emulsify the Sauce: This is the most important part. Pour in about 120ml of the reserved pasta water and toss the pasta vigorously for a minute or two. What works best for me is to keep tossing until the sauce thickens, becomes glossy, and clings beautifully to every strand. If it looks too thick, add another splash of pasta water until you reach your desired consistency.

Finish and Serve: Stir through the chopped fresh parsley. Serve immediately in warm bowls, with an extra grating of Parmesan cheese and a final crack of black pepper on top.