Cut pork into 3 chunks and place in a large saucepan. Add onion, garlic, bay leaves, and salt and cover with water. Bring to a boil; skim foam from surface. Reduce heat, cover, and simmer for 1 hour. Remove pork and let cool. Strain broth and reserve.
Bring a pot of water to a boil while pork is cooking. Add tomatoes, arbol chiles, and guajillo chiles, and boil until chiles are soft, about 5 minutes. Remove with a slotted spoon, reserving cooking water, and allow to cool.
Place corn husks in a bowl, cover with boiling water, and soak for 30 to 60 minutes. Drain, place on a work surface, and cover with a clean, damp towel.
Combine tomatoes, 1/2 cup cooking water, chiles de arbol, guajillo chiles, and cornstarch in a blender; blend until smooth. Strain tomato sauce through a fine-mesh sieve.
Beat lard with an electric mixer in a large bowl until fluffy. Combine masa, 1 cup reserved pork broth, 1 tablespoon salt, and baking powder in a separate bowl and mix until smooth. Add masa mixture to lard and mix until it has a smooth cookie dough consistency. Test if the masa is ready by dropping a small ball of masa into a glass of cold water; if it floats, it's ready, if not, keep beating for a little longer.
Shred cooled pork with 2 forks.
Select 1 wide corn husk or 2 small ones. Spread about 2 tablespoons masa mixture onto the the corn husk, filling it up to 2 inches from the bottom and 1/4 inch from the top. Add 1 tablespoon of the tomato sauce and pork down the center of the masa mixture. Fold sides of husk together, 1 over the other. Fold the bottom of the husk over the seam of the 2 folded sides. Repeat with remaining husks.
Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add tamales with the open side up and cook until filling is heated through and separates from the husk, about 1 hour. Let tamales rest for 15 minutes before serving.
Masa can be found at select Mexican markets. If not available, you can make it with instant corn masa flour: mix 4 cups of masa harina with 3 cups of pork broth, until moist and pliable. If necessary, add more broth.
Per Serving: 343 calories;20.3 g fat;
30 g carbohydrates;
9.7 g protein;
33 mg cholesterol;
644 mg sodium.