Fill a large bowl or sink with hot water.
Submerge corn husks for 30–45 minutes until pliable. Weigh them down with a plate so they stay underwater.
Rinse each husk to remove any dust. Drain and keep covered with a damp towel.
Add pork, onion, garlic, bay leaves, salt, cumin, and water to a large pot.
Bring to a gentle boil, then lower to a simmer for 1.5–2 hours until the pork shreds easily.
Reserve at least 4 cups of cooking liquid for sauce and masa. Shred pork with two forks and set aside.
Warm a dry skillet over medium. Lightly toast guajillo, ancho, and pasilla chiles for 20–30 seconds per side until fragrant. Do not burn.
Place chiles in a bowl and cover with hot water. Soak 15 minutes until softened, then drain.
Blend softened chiles with onion, garlic, oregano, cumin, vinegar, sugar, and 2 cups pork liquid until very smooth.
Heat oil in a saucepan. Strain the blended sauce through a fine-mesh sieve into the pan. Simmer 10–12 minutes and season with salt.
Toss shredded pork with enough sauce to coat generously, saving extra sauce for serving.
Whisk masa harina, baking powder, and salt in a large bowl.
Beat lard in a stand mixer or bowl until light and fluffy, about 2–3 minutes.
Add the dry ingredients to the lard in batches, alternating with warm pork liquid. Mix until the dough feels like soft peanut butter and holds a spoon upright. Add more liquid as needed.
Test: Drop a small piece of masa into cold water. If it floats, you’re good. If it sinks, beat in a bit more fat or liquid and retest.
Pat a husk dry and place the smooth side facing up, narrow end pointing away from you.
Spread 2–3 tablespoons masa into a 4–5 inch square, leaving a border at the sides and bottom.
Spoon 1–2 tablespoons pork in chile sauce down the center.
Fold the sides over the filling to meet in the center, then fold the narrow end up. Tie with a thin husk strip if you like.
Repeat until you use all masa and filling.
Set a steamer basket in a large pot. Add water just below the basket.
Stand tamales upright, open end up, snug but not packed. Cover with leftover husks and a lid.
Bring water to a simmer and steam for 60–75 minutes. Check occasionally and add hot water as needed so the pot never dries out.
Test for doneness: The husk should peel away cleanly and the masa should feel set and fluffy. If sticky, steam 10–15 minutes more.
