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 prawns and toss for 2 minutes.
Pour the tamarind sauce over the prawns and cook for another 3 minutes.
Add 2 tbsps basil leaves and chili and cook for another 2 minutes or until the prawns are cooked through.
Remove the prawns and transfer them to a serving plate.
Let the sauce cook on high heat for 10 minutes until it becomes thicker.
Pour the sauce over the prawns. Garnish with more basil and chili and serve immediately.