Combine sauce ingredients in bowl and set aside.
In a medium sized pot combine stock with coconut milk. Bring to boil. Add galangal, lemongrass, one Thai chilies, shallots, and kaffir lime leaves. Add sugar and salt.
Simmer for a couple of minutes, and then add chicken, simmer about 2 minutes, add mushrooms. Turn down the heat and continue to simmer until chicken is cooked, about 5-7 minutes. Add prepared sauce and serve.