Place pot roast in a large pot, along with onion, garlic, and enough water to cover. Simmer until meat is cooked through.
Remove stems and seeds from chiles. Put chiles and 2 cups water in a saucepan and simmer, uncovered for 20 minutes. Remove from heat and allow to cool. Transfer chile mixture to a blender and blend until smooth.Strain mixture, add 1 1/2 tsps. salt, and set aside. Shred cooked beef and add 1 cup of chile mixture.
Soak corn husks in warm water.
In large mixing bowl, beat lard and 1 tbsp broth until fluffy.
Combine masa harina, baking powder, and salt and stir into lard mixture, adding broth as necessary until mixture reaches the consistency of creamy peanut butter.
Spread dough out over husk to about 1/2 - 1 inch width and 1/4 to 1/2 inch thickness. Place one tablespoon of meat mixture over dough, and spread from top to bottom.
Fold in sides of husk towards center. Repeat steps 6 & 7 until all dough is used.
Place in a steamer pot and steam for 1 hour.
Remove from husks and drizzle remaining chile sauce over.