Heat a wide 4 - 5 quart heavy pot over medium heat until hot, then toast the pumpkin seeds, stirring constantly, until aromatic and starting to pop and crackle, about 4 to 5 minutes. Transfer to a plate and let cool.
½ lb pumpkin seeds
Place the tomatillos, onion, garlic, peppers, epazote, salt, ½ cup cilantro, and ½ cup water in a blender and purée until very smooth.
1 lb tomatillos, ½ cup onion, 8 cloves garlic, 2 serrano chiles, 2 tbsp epazote, 1 tbsp Kosher salt, 1 cup cilantro
Use a spice grinder (or coffee bean grinder) to grind pumpkin seeds to a fine powder in small batches. Place the pulverized seeds in a bowl and then stir together with 2 cups water. Set aside.
Heat oil in a large skillet over medium heat until hot but not smoking, then carefully pour in the pumpkin seed mixture, stirring constantly, until thickened to a slightly thick consistency, about 10 to 15 minutes.
¼ cup corn oil
Add tomatillo puree and simmer, stirring often for 2 minutes.
Stir in the 1½ cups water, and simmer, uncovered, stirring frequently, for 10 to 15 minutes, until smooth and slightly thickened.
Just before using, puree the remaining ½ cup of cilantro with ½ cup of water in your blender until smooth and add to the sauce. If necessary, thin with a little more water and return to a simmer. Use at once or store in a container with a lid in the refrigerator for up to 5 days.