Put most of the shallots (reserving 2 tbsps), ginger, garlic, peppercorns, turmeric, chilli and coriander powders into a mortar or blender and pound or blend to a coarse paste without using any water. In a bowl, mix the spice paste, grated mango and salt with the cleaned prawns. Cover and marinade for 2 hours in the refrigerator. After the two hours, transfer the mixture to a large pan and heat it over low heat. Cover the pan and gently bring it to a simmer, stirring once or twice.