First, make the sauce. Take the tamarind pulp (a block about 1 x 2 in (2.5 x 5 cm) and mix it with ½ cup boiling water (you can add a little more if needed to dissolve the paste). Break up the pulp in the hot water, and then press the mixture through a fine-meshed strainer. Discard the solids. To the liquid tamarind concentrate you just made, add the sugar, fish sauce, Thai black soy sauce, Thai sweet soy sauce (if using), and white pepper. Set aside.
Soak the pad thai noodles in hot water for about 20 minutes, and drain in a colander. If the noodles are in really long strands, you will want to cut them into 10- to 12-inch lengths to make stir-frying easier.
Marinate the sliced chicken by combining it with 1 teaspoon each of Thai thin soy sauce, cornstarch, and water. Set aside.
Next, prepare the dried shrimp, mincing them down into a coarse powder (we used a food processor). Prepare the garlic, shallots/red onion, preserved Chinese mustard stems (zha cai), eggs, mung bean sprouts, garlic chives, and peanuts. You want to have everything ready to go before you turn on the stove.
Now you’re ready to cook! Heat 2 tablespoons of oil in your wok over high heat until just smoking. Add the chicken, and sear until golden and just cooked through. Remove from the wok and set aside.
Add another 3 tablespoons of oil to the wok. Over medium heat, add the shrimp powder. Fry until fragrant and crisp, 2 minutes. Add the garlic, and cook for 30 seconds. Add the shallots and zha cai. Stir-fry for another 30 seconds.
Turn the heat up to high, and add the noodles and sauce. Stir-fry to combine, lifting the noodles with your wok spatula to spread them out and break them up.
Make a space on the side of the wok, pushing the noodles to one side. Drizzle 1 more tablespoon of oil in the open space, and pour in the beaten eggs. Use your spatula to fold them gently, scrambling them without breaking up the egg too much. When the eggs are about 70% done, stir-fry to distribute them into the noodles.
Next, add the bean sprouts and the chives. Stir-fry to combine, letting the chives wilt. Add the chicken back in, and stir-fry to combine until everything is incorporated. Plate, top with the crushed peanuts, and serve!