Trim fat from pork. Cut into chunks and place in a large saucepan. Add onion, garlic, bay leaf and salt. Cover with water and bring to the boil; skim foam from surface. Reduce heat, cover and simmer for 1 hour. Let meat cool slightly and shred with two forks. Reserve some of the cooking liquid.
Meanwhile, lightly toast both sides of chillies on a frying pan over medium heat, being careful not to burn them because they get bitter. Soak toasted chillies in boiling water for 30 minutes, until soft.
Grill onion and 3 cloves in a dry frying pan till lightly charred. Blend with soaked chillies, bay leaf, thyme, marjoram and a couple tablespoons of water (only enough to blend with ease). The sauce should be thick. Strain into a saucepan over medium high heat with 2 tablespoons hot lard; simmer sauce for 5 minutes and season with salt.
Stir in pork meat and simmer for 5 minutes.
In a medium bowl, beat lard until fluffy and white. Gradually add corn masa dough and continue beating until smooth. If necessary, add some of the pork cooking liquid, a little bit at a time. Add 1 tablespoon salt and baking powder; continue beating until fluffy. (Take a little ball of dough and drop it into a glass with cold water. If it floats, it's ready, if not, continue beating).
Grease with lard a 25cm baking dish. Spread half of the tamale dough on the dish. Add the pork meat with sauce and cover with the remaining dough. Cover dish with foil, sealing edges the best you can.
Boil water in a large pot with a steamer rack. Place the dish with tamale on the rack (water shouldn't touch dish) and cover. Steam tamale until dough pulls away from the edge of dish, about 1 hour.