Cook pasta until al dente. Heat 1 tbsp of the oil in a pan, add tomatoes. Bring to boil and cook for 4-6 mins, stirring and crushing until slightly reduced.
In another pan, gently fry the chilli, anchovies and garlic in the remaining oil and butter (if using) for 1-2 mins, mashing the anchovies.
Add the tomatoes, olives and capers to the chilli, anchovies and garlic. Cook for 2-3 mins, stirring.
Drain the pasta and add to the sauce. Add the parsley, season and stir well to mix.