Bring the water to a boil in a pot. Add the sliced galangal and lightly crushed lemongrass. Once boiling, add the straw mushrooms and shrimp heads and shells for flavor and richness. Reduce to medium heat and simmer until the broth is fragrant and boiling again.

Add the crushed chilies, Thai chili paste, and fish sauce. Stir until well combined. When it comes to a boil again, add the shrimp and cook just until they turn pink. Pour in the evaporated milk and heat until it just starts to boil again. Tear the kaffir lime leaves and add them to the pot. Turn off the heat and stir in the lime juice.

Ladle into bowls and serve hot.