In a 12-inch skillet, combine the sugar and 2 tablespoons water. Cook over medium-high, occasionally swirling the pan, until the caramel is mahogany in color and smokes lightly. Off heat, add the fish sauce. Bring to a boil, stirring to dissolve any clumps, then add the chilies, shallots, ginger and ½ teaspoon pepper. Bring to a simmer and cook, stirring, for about 30 seconds. Add the shrimp and cook over high, stirring, just until opaque. Remove and discard the lemon grass, if used.