With a sharp knife, peel the pumpkin and cut into quarters. Scoop out the seeds with a teaspoon and discard. Cut the flesh into chunks (about 3/4 inch) thick and set aside.
Put the garlic, shallots, shrimp paste, lemongrass, green chiles and salt to taste in a mortar. Drain the dried shrimp, discarding the soaking liquid, and add them, then use a pestle to grind the mixture to a paste.
An alternative would be to place the ingredients in a food processor and process to a paste.
In a large pan, bring the chicken stock to a boil. Add the ground paste and stilr well to dissolve.
Add the pumpkin chunks and bring to a simmer. Simmer for 10-15 minutes or until the pumpkin is tender.
Stir in the coconut cream, then bring the soup back to simmer. Be careful not to let it boil. Add the fish sauce, sugar and ground black pepper to taste.
Add the shrimp and cook for 2-3 minutes.
Serve in warmed soup bowls and garnish with chiles and basil leaves.