For the aromatic broth: Bring the water to a gentle simmer in a large saucepan over high heat. Whisk in the chicken powder, then add the galangal, lemongrass, Thai chiles and lime leaves. Lower the heat and simmer, uncovered, 20 to 30 minutes, until the broth is fragrant and lightly golden. (Do not boil — a gentle simmer keeps the flavors clean.) Strain out the aromatics if you like; I prefer to leave them in for an authentic presentation, but if you do be sure to warn your guests about them so they don’t chew on them.
To finish: Add the mushrooms and return the broth to a simmer for 2 minutes, until heated through. Add the shrimp and cook 1 to 2 minutes, just until they turn opaque. Turn the heat to low. Stir in 4 tablespoons of the fish sauce, 4 tablespoons of the lime juice and the nam prik pao (chili paste). Use a whisk to help the paste fully dissolve and distribute in the broth. Simmer 30 to 60 seconds more, just to meld.
Taste the soup and balance it as needed: Add more fish sauce for salt, more lime juice for acidity or a touch more chili paste for heat and sweetness. Ladle into bowls and garnish with cilantro and scallion. Serve hot.
