In a large microwave-safe measuring cup or bowl, combine the brown sugar, water, tamarind paste, oyster sauce, fish sauce, and sweet soy sauce. Microwave on high for 1 minute, stir, then microwave for another minute. Continue heating in 30-second intervals, stirring after each, until the sugar has completely dissolved and the sauce is smooth (about 2–3 minutes total). Set aside.

Place the rice noodles in a large bowl and cover with hot (not boiling) water. Soak for 10–15 minutes, stirring occasionally, until the noodles are flexible but still slightly firm. Drain well.

Toss the sliced chicken thighs with the cornstarch, salt, pepper, and garlic powder until evenly coated.

Heat 1 tablespoon of oil in a large wok or skillet over high heat. Cook the chicken for 5–6 minutes, stirring frequently, until browned and cooked through. Transfer to a plate.

Add the remaining tablespoon of oil to the pan. Add the shallot and cook for 30 seconds until fragrant.

Push everything to one side of the pan and crack in the eggs. Scramble until just set.

Add the drained noodles and pour in ½ cup of the prepared Pad Thai sauce. Toss continuously for 2–3 minutes until the noodles become tender and evenly coated. If the noodles seem dry, add another tablespoon or two of sauce or a splash of water.

Return the chicken to the pan. Add the bean sprouts and garlic chives and toss everything together for 30–60 seconds, just until the vegetables begin to soften while still retaining some crunch.

Divide between four bowls and top with the chopped peanuts. Serve immediately with fresh lime wedges.