Mixed Mushroom Tamales

Ingredients:
36 large dried corn husks (3 oz), separated and any damaged husks discarded
1/2 oz dried porcini mushrooms (1/2 cup)
2 1/2 cups very hot water
2 sticks (1 cup) unsalted butter, softened
1/2 small onion, finely chopped (1/4 cup)
1 tablespoon minced garlic
1 lb mixed fresh mushrooms such as shiitake caps, white button, and cremini, coarsely chopped (4 1/2 cups)
1 teaspoon dried epazote (optional), crumbled
1/2 teaspoon black pepper
2 teaspoons salt
2 cups fine-ground masa harina (dry corn masa; 9 1/4 oz)
1/2 teaspoon baking powder
1 teaspoon sugar
a large pasta pot with a perforated steamer/colander insert or a deep pot and a large collapsible vegetable steamer

Instructions:
Cover husks with hot water by 2 inches in a large bowl and soak, kept submerged with an inverted plate, turning husks occasionally, until soft, about 30 minutes. Rinse husks, 1 at a time, under running water. Pile 24 of largest husks on a plate and cover with a dampened kitchen towel. Tear some of remaining husks lengthwise into 24 (1/2-inch-wide) strips to use as ties (keep damp as well).
Cover porcini with 1 cup very hot water in a small bowl and soak 30 minutes. Lift out porcini, squeezing liquid back into bowl (reserve liquid), then rinse mushrooms to remove any grit. Coarsely chop porcini. Pour soaking liquid through a paper-towel-lined sieve into a glass measure and reserve.
Heat 1/2 stick butter in a 12-inch heavy nonstick skillet over high heat until foam subsides, then sauté onion and garlic, stirring, 1 minute. Add mushrooms (including porcini) and epazote (if using) and sauté, stirring occasionally, until liquid is released, about 3 minutes.
Add porcini soaking liquid and simmer, stirring occasionally, until most of liquid is evaporated and mushrooms are slightly browned, 3 to 5 minutes, then sprinkle with pepper and 1/2 teaspoon salt. Transfer to a bowl to cool.
Beat remaining 1 1/2 sticks butter with an electric mixer at medium-high speed until light and fluffy, about 30 seconds. Sift masa harina with baking powder, sugar, and remaining 1 1/2 teaspoons salt into a bowl. Stir in 1 1/2 cups very hot water until a thick paste forms.
Beat masa mixture into butter in 3 batches, beating until smooth after each addition. Reduce speed to low and mix in mushroom mixture until just combined.
Put 1 husk on a work surface, pointed end closest to you, and, spreading it flat, mound 3 tablespoons filling in center and flatten slightly into a rough oval (about 1/2 inch thick) with back of a spoon, leaving a 1-inch border on both sides. Bring pointed end of husk up over mound of filling to cover, and fold sides of husk over filling to enclose. Gather together open end of husk at top of filling, creating a flat pouch, and tie with a corn-husk strip. Assemble 23 more tamales in same manner.
Arrange tamales upright in 1 layer in steamer insert so they resemble falling dominoes in rows. Set steamer over boiling water in pot and cover with a folded kitchen towel (towel absorbs condensation so tamales don't get soggy). Steam tamales, tightly covered with a lid, adding more water as necessary, until filling is tender, about 30 minutes. To check for doneness, open 1 steamed tamale and if any part of filling is still gummy, steam 5 to 10 minutes more.