Wash the shrimp. Separate the heads from the body and save for use in the broth. Take the shells off and devein the shrimp.
Place the shrimp heads in 4 cups water and simmer for 10 minutes. Strain the shrimp heads out and then add the roasted chili paste to the broth.
Next add the lemongrass, lime leaves, and bruised chilis to the broth. Simmer for a few more minutes to allow the herbs to release their flavors (at least 5 minutes).
Add the mushrooms and allow to cook through. Then add the shrimp and cook until just done.
Add fish sauce and lime juice. Taste the broth and adjust as needed.