Make the filling: Place the black beans, garlic, chili powder, dried chilies, and epazote in a large pot and cover with fresh water by 2 inches. Bring to the boil and cook for one half hour. Add the 1 tablespoon salt and continue cooking until very tender. Drain. Transfer the solids to the work bowl of a food processor, add the tomatillos, and coarsely chop.
Make the tamale dough: Mix together the salt and masa harina in the bowl of a mixer. Slowly add the hot liquid, stirring to obtain a soft, moist dough. Beat the dough with the paddle attachment for five minutes to aerate the dough as much as possible. Sprinkle in the baking powder and beat 1 minute more. (Contemporary recipes contain lard, which results in a lighter product.)
Assemble the tamales: Place a husk flat on the counter horizontally in front of you. Spread about 2 tablespoons of tamale dough in a thin layer over the husk, leaving a border around the edges. Top with about 1 tablespoon bean filling. Pull up the husks on the right and left edges and nudge the dough so that it folds over the filling. Then do the same with the top and bottom edges of the husk, so that the dough encases the filling and forms a rectangular packet. Fold the right and left sides of the husk over the dough packet, then fold the bottom of the husk up and the top of the husk down. Keep folding to make a neat package. Repeat with remaining husks.
Place the tamales in a covered steamer and steam for 45 minutes, making sure that the steamer does not dry out. Serve immediately, or reheat by steaming.