Heat oven to 160C. Place butter and olive oil in a large heavy-based casserole dish over a low heat. Add the onion and celery and fry for 4 minutes, or until soft and translucent. Add the garlic, oregano and chilli flakes and fry for a further 1 minute.
Using a sharp knife, make an insert into the end of each sausage. Squeeze the sausage meat out of the casings and add to the casserole dish. Increase the heat to medium and fry for 4-5 minutes, or until the sausage meat begins to brown. Add the tomato paste, tomatoes, stock and a generous pinch of salt and pepper.
Bring to a simmer, then place in the oven and bake for 2 hours, or until liquid has reduced by half.
Return casserole dish to stove top, add the vodka and simmer for 3 minutes. Add the cream and parmesan and gently cook for 5 minutes.
Bring a large saucepan of salted water to the boil. Cook pasta for 7 minutes, or until al dente. Drain and add to casserole dish just before serving. Garnish with basil leaves and extra parmesan.