Step 1

Place the noodles in a large bowl and add hot water to cover (the water should feel hot to the touch, but should not be scalding). Let stand, stirring once or twice to separate any strands that are sticking together, for about 30 minutes; the noodles will become pliable but will not fully soften.

Step 2

Meanwhile, in a small bowl, combine the tamarind pulp and boiling water; stir with a fork to break up the pulp. Cover and let stand for about 30 minutes. Strain the mixture through a fine-mesh sieve set over another small bowl; press on the solids and be sure to scrape the underside of the sieve to collect the pulp that clings; you should have about ¼ cup. Wipe out the small bowl used to hydrate the tamarind, then measure 3 tablespoons of the strained tamarind into it (reserve the remainder for another use). Add the sugar, oyster sauce, soy sauce and fish sauce. Stir until the sugar dissolves; place near the stove.

Step 3

Drain the noodles in a colander. Shake the colander to remove excess water and set near the stove. In a medium bowl, toss together the bean sprouts, chives and peanuts; also set near the stove.

Step 4

In a 12- to 14-inch wok over high, heat 1 tablespoon of the oil until smoking, swirling to coat. Add the shrimp and cook, stirring, until just beginning to curl and turn pink, 1 to 2 minutes; the shrimp will not be fully cooked. Transfer to a large plate; set aside. Wipe out the wok.

Step 5

Return the wok to high and heat 2 tablespoons of the remaining oil until smoking, swirling to coat. Add the shallot, garlic and pepper flakes; cook, stirring, until fragrant and lightly browned, 20 to 30 seconds. Add the eggs (they will immediately puff) and cook, stirring from the edges inward, until the curds are barely set and shiny, 20 to 30 seconds.

Step 6

Add half of the noodles. Cook, stirring, tossing and moving the noodles in a circular motion against the sides of the wok while also breaking up the eggs, until the noodles are dry, sizzling and no longer stark white in color, 1 to 1½ minutes. Drizzle the remaining 1 tablespoon of oil down the sides of the wok and add the remaining noodles; cook in the same way until the mixture is once again dry and sizzling.

Step 7

Pour half of the sauce mixture down the sides of the wok; it should bubble immediately and begin to thicken. Cook, tossing and moving the noodles in a circular motion against the sides of the wok, until the liquid is absorbed, about 30 seconds. Add the remaining sauce mixture and cook in the same way.

Step 8

Add the shrimp (discard any accumulated juices) and half of the bean sprout mixture. Cook, stirring, until the shrimp are opaque throughout and the sprouts are just wilted, 1 to 2 minutes. Taste the noodles; if they are still too firm, drizzle in water 2 tablespoons at a time and cook, stirring, until the noodles are tender. Toss in the remaining sprout mixture. Transfer to a platter and serve with lime wedges and chilies in vinegar (if using).

Step 9

To make Pad Thai with Tofu: Cut 8 ounces firm or extra-firm tofu into ½- to ¾-inch cubes. Place in a single layer on a plate lined with a double thickness of paper towels. Cover with additional paper towels, place another plate on top, then set a few cans or jars on top as weights; let stand while you soak the noodles, prepare the tamarind and mix the sauce ingredients. Follow the recipe, substituting the tofu for the shrimp and stir-frying until the tofu is golden brown on all sides, about 3 minutes, setting it aside, then returning it to the wok as with the shrimp.

Step 10

Pad Thai with Ground Pork: Follow the recipe, omitting the shrimp. When the shallot, garlic and pepper flakes are fragrant and lightly browned, add 8 ounces ground pork; cook, stirring and breaking the pork into small bits, until the meat is lightly browned, 2 to 3 minutes. Make a clearing in the center of the mixture and add the eggs to the clearing, then continue with the recipe.