In a large bowl combine the 2 cups flour and salt. Gradually add boiling water, stirring constantly. Stir in 1/4 cup cold water. Turn out onto a lightly floured surface; let stand until cool. Knead in enough of the 3 tablespoons flour to make a moderately stiff dough that is smooth and elastic (6 to 8 minutes total). Shape into a ball. Cover and let rest for 20 minutes.
Soak mushrooms for 30 minutes in enough warm water to cover. Rinse well; squeeze. Chop finely, discarding stems. Thaw shrimp, if frozen. Chop finely. For filling, combine water chestnuts, cornstarch, onion, rice wine, soy sauce, ginger, sesame oil, and pepper. Add pork; mix well.
Divide the dough in half. On a lightly floured surface, roll each half to a 1/16-inch thickness. Cut into 3-inch rounds, rerolling dough as needed. Spoon 1 slightly rounded teaspoon of filling onto each round. Bring opposite sides over filling; pinch together only in center. Pinch remaining sides together only in the center. Repeat with remaining rounds and filling. Enlarge openings atop dumplings with a chopstick. Place on a lightly floured baking sheet; cover. Chill dumplings in the freezer for 20 minutes. Finely chop the carrot and egg. Fill openings with carrot, egg, mushrooms, and shrimp. In a wok place a greased steamer rack over water. Bring water to boiling over high heat. Place dumplings on rack so sides do not touch. (Cover and chill extra dumplings.) Cover and steam about 15 minutes or until no pink remains in pork. Repeat with remaining dumplings. Serve with Chili Dipping Sauce. Makes 10 appetizer servings.