Grilled Shrimp with Tamarind Sauce

Name: Grilled Shrimp with Tamarind Sauce
Ingredients: 1 1/2 lb jumbo (8 to 12 per lb) shrimp; 3 teaspoons sea salt dissolved in 11/2 cups water; 1 tablespoon tamarind from a pliable block; 1/3 cup water; 1 tablespoon peanut oil; 1 tablespoon chopped cleaned fresh cilantro roots and/or stems, pounded to a paste; 1 tablespoon minced garlic; 1 tablespoon Sriracha or other Asian hot chile sauce; 1 1/2 tablespoons palm sugar, or to taste; 1 tablespoon Asian fish sauce (preferably Thai naam pla), or to taste; Garnish: 2 fresh serrano or jalapeño chiles, seeded, halved lengthwise, and finely julienned crosswise; and fresh cilantro sprigs