Cook the massaman paste in the thick bit of the coconut milk for a few minutes until it becomes fragrant. Add the rest of the coconut milk and simmer for a minute. Add the potatoes, cinnamon, cardamom and simmer for 15-20 minutes until the potatoes are tender. Add the prawns, sugar, lime and fish sauce, and cook for 3-4 minutes until the prawns are just cooked and pink.
Serve the curry sprinkled with peanuts (if using), chilli and basil.