1. Toast the pumpkin seeds in a frying pan over high heat, stirring frequently. (Avoid browning to maintain the sauce's trademark green color.) Seperate ¼ cup for the garnish. Toast the sesame seeds in the same manner.
2. Place the tomatillos, chile, onion and garlic on a tray under the broiler until soft. About 5 minutes on each side.
3. Place all the ingredients, except for the shrimp, in a blender. Puree until smooth.
4. Pour the sauce into a stockpot over high heat and bring to a boil. Lower to medium heat and let simmer for about 30 minutes.
5. Add the shrimp to the Pipian and cook for about 3 minutes or until done.
6. Spoon over white rice. Garnish with the remaining ¼ cup of toasted pumpkin seeds.
Pipian freezes well, so make a double batch and save some for a quick meal when there's no time to cook.