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 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.