How to Make It
Put the noodles in a large bowl and cover with very hot water. Let soak until just pliable, about 15 minutes (or according to package directions). Drain the noodles in a colander, shaking off any excess water.
Meanwhile, in a small bowl, whisk the soy sauce with the brown sugar, tamarind paste and Sriracha.
In a large nonstick wok or skillet, heat the oil until shimmering. Add the shallots and mushrooms and cook over high heat, stirring occasionally, until lightly browned, about 5 minutes. Add the garlic and carrots and stir-fry until soft, about 2 minutes. Add the noodles and the 4 scallions and stir-fry until heated through, about 2 minutes.
Scrape the noodles to one side of the pan and add the eggs. Cook, stirring occasionally, until nearly set, about 1 minute. Toss everything together. Add the sauce and stir-fry until the noodles are evenly coated and the sauce has thickened slightly, 2 to 3 minutes. Transfer the pad Thai to a platter. Garnish with the bean sprouts and thinly sliced scallions, then serve.
Tamarind paste is available from Indian, Asian and Latin markets.