Preheat the oven to 180°C/350°F/gas 4. Place the salmon fillets, skin-side down, in an ovenproof dish. Rub the butter over the salmon fillets and season well. Cover the dish with foil and place it in the oven for 15 minutes or until the salmon is cooked through.
Meanwhile, cook the spaghetti according to the instructions on the package.
Heat the oil in a large pan over a low heat. Add the garlic and cook for 2 minutes. Stir in the tomatoes, capers and Parmesan. Tear the basil leaves and stir them into the pan. Toss in the cooked pasta and mix well. Divide the pasta between warmed serving bowls and top with the cooked salmon. Divide the rocket evenly between the bowls and serve.