In medium bowl, muddle grapes until mashed and in small pieces. Transfer to two lowball glasses and add ice almost to top of glasses.
In shaker or small bowl, combine whiskey, lemon juice and tamarind paste; shake or stir. Season with a few drops of bitters, to taste. Pour into glasses over grapes and stir briskly to chill and dilute slightly.
Notes
Nutritional information per serving: 140 calories; 0 g protein; 11 g carbohydrates; 0 g fat (0% calories from fat); 0 g saturated fat (0% calories from saturated fat); 0 mg cholesterol; 0 mg sodium; 1 g fiber.