Soak rice noodles in warm water until flexible, 20–25 minutes. Drain well and set aside.
Whisk tamarind, fish sauce, brown sugar, soy sauce, lime juice, water, and red pepper flakes until smooth.
Heat a large skillet or wok over medium-high. Add 1 1/2 tbsp oil and swirl to coat.
Stir-fry chicken until just cooked through, 3–4 minutes. Transfer to a plate.
Add remaining oil. Sauté garlic until fragrant, about 20 seconds.
Pour in beaten eggs and scramble gently until just set. Push to one side of the pan.
Add drained noodles and sauce. Toss constantly until coated and tender, 2–3 minutes.
Return chicken. Add bean sprouts and most green onions. Stir-fry 1 minute to heat through.
Plate and top with peanuts, remaining green onions, and cilantro. Serve right away.