Heat the butter in a saucepan, add the pancetta and cook for about 3 minutes until golden and crispy. Remove from the pan and keep warm. Stir in the rosemary, then remove from the heat and pour over the vodka. Return to a high heat and cook until the vodka has reduced down to 1 tablespoon. Pour over the tomato pasta sauce, reduce the heat and simmer for 10 minutes.
Meanwhile, cook the pasta in a large saucepan of salted boiling water according to the pack instructions until al dente.
Remove the rosemary from the sauce, then stir in the cream. Drain the pasta, reserving a little of the cooking water, and return to the pan. Stir through the sauce, adding a little cooking water to loosen if needed, and season.
Spoon into serving bowls, top with the pancetta slices and serve scattered with the Parmesan.