Remove the heads from the prawns and place them into a saucepan with the boiling water and half the ginger. Bring to the boil and cook for 10-12 minutes. Strain through a sieve into a bowl, discarding the prawn heads and ginger.
Return the cooking liquid to the saucepan and add the coconut milk and one tablespoonful of the soy sauce. Add the prawns and simmer gently until they turn pink. Remove the prawns from the liquid using a slotted spoon and set aside.
Place the remaining soy sauce and ginger, along with the mango, sesame oil, garlic, lime juice, sesame seeds and half the coriander into a small food processor and blend until smooth.
Pour this mixture into the coconut milk mixture and bring to the boil. Return the cooked prawns to the liquid and warm through.
To serve, pour the broth into a serving bowl, add the cooked noodles and garnish with the rest of the fresh coriander.