To make the tamarind water, soak the tamarind paste in boiling water for 5 minutes breaking up any chunks with a spoon. Set aside until cool enough to handle. Pass the mixture through a sieve to get the tamarind water.
To make the tamarind sauce, in a bowl whisk together tamarind water, fish sauce, and sugar until well combined.
Add the oil, garlic, and shallots to a hot pan. Saute over medium heat for 2-3 minutes or until fragrant.
Increase the heat to medium-high. Add the peppercorns and razor clams and toss for 1 minute.
Pour the tamarind sauce over the prawns. Put the lid on the pan and let the razor clams cook for 2-3 minutes until they open. Transfer the clams to a serving platter.
Let the sauce cook on high heat for 10 minutes until it becomes thicker. Add 2 tbsps basil leaves and chilis and cook for another 2 minutes for the basil and chilis to release their flavors.
Pour the sauce over the prawns. Garnish with more basil and chilis and serve immediately.