Cook penne in a saucepan of well-salted boiling water until al dente (10-14 minutes), then drain, reserving a little pasta water.
Meanwhile, for toasted crumbs, combine ingredients except garlic in a large deep frying pan over medium heat until toasted, stirring in garlic in the last minute (4-5 minutes). Transfer
to a plate and wipe out pan.
Heat olive oil in pan over medium heat, sauté garlic and rosemary until fragrant (1-2 minutes), then add prawns and chillies and stir until just starting to turn opaque (1 minute). Add polpa, bring to a simmer, season to taste and simmer until sauce is well flavoured, and prawns are cooked (2-3 minutes).
Add pasta to sauce, toss to combine and loosen with a little pasta water if necessary. Serve scattered with toasted crumbs and parsley.