1. Peel and devein the shrimp, leaving the tails on, set aside.
2. Place the shells in a saucepan with water and salt over medium heat.
3. Cook for 20 minutes to make a light broth.
4. Strain, reserve the broth, discard the shells.
5. Allow the broth to cool, and then return to the saucepan over low heat.
6. Add the shrimp and cook for about three minutes, tossing, until they are pink and barelay opaque.
7. Strain, reserving the broth in a bowl so that it can cool.
8. Place the pumpkin seeds in a large, dry skillet over medium heat.
9. Cook for about eight minutes, stirring, until they begin to swell and pop about, do not let them.
10. Let cool, then place 1/3 of them in a blender with about a third of the cooled broth.
11. Puree until smooth, then add another third of the shrimp broth and another third of the seeds, along with the onion, cilantro and chilli peppers.
12. Puree until smooth, then add the remaining broth and seeds.
13. Puree again, to form a green sauce.
14. Melt butter in a saucepan over low heat.
15. Add the blended sauce and cook for five minutes, add the sour cream and adjust the seasoning, heat, stirring.
16. Serve hot.