Shrimp in pumpkin seed sauce

1 1/2 pounds medium shrimp, unshelled
2 1/2 cups cold water
1 teaspoon salt or to taste
Freshly ground pepper
1 cup unroasted, unsalted, hulled pumpkin seeds
8 sprigs cilantro
3 serrano chiles or any fresh green chiles wiped clean
1/2 small white onion
1 tablespoon unsalted butter
2/3 cup sour cream

1. Shell and devein shrimp and set aside.
2. Put shells, tails and heads, if any, into saucepan. Add water with salt to taste and cook over medium heat about 20 minutes to make light broth. Strain and discard shells, reserving cooking liquid. Cool slightly.
3. Add shrimp and cook over heat about 3 minutes or until just turning opaque. Drain shrimp, reserving broth.
4. Toast pumpkin seeds lightly in heavy ungreased skillet, stirring frequently, until they begin to swell and start to pop about. Do not let brown. Cool, then grind finely in spice grinder. (Alternatively add to blender with broth in next step, but sauce will not be smooth.)
5. Place shrimp broth, pumpkin seeds, cilantro, chiles and onion in blender or food processor. Blend until smooth.
6. Melt butter in heavy saucepan Add blended pumpkin seed sauce and cook over very low heat stirring and scraping bottom of pan constantly, about 10 minutes. Add shrimp and heat through another 5 minutes. Sauce should be of medium consistency.