In a food processor, pulse the masa harina with the baking powder, and salt. Add the water and pulse to moisten the masa harina. Add the vegetable shortening in clumps and drizzle with the chicken stock. Process until smooth and evenly blended, scraping the side of the bowl occasionally. Transfer the dough to a large bowl and stir in the shredded chicken, shredded cheese, chopped cilantro, and scallions.

Tear twelve 12-inch-square sheets of plastic wrap. Working with 6 at a time, spoon a scant 1/2 cup of the tamale mixture onto the center of each sheet. With lightly moistened hands, press the tamales into 4-by-2-inch rectangles, about 1/2 inch thick. Fold up the squares of plastic around the tamales, twist the ends securely and then fold them under to make packets. Repeat to form the remaining tamales.

Arrange the tamales seam side down in the bottom of a large bamboo (or other) steamer. Steam over 2 inches of boiling water for 25 minutes. Let the tamales cool slightly, then carefully transfer them to a large platter: Cut the ends of the plastic-wrapped packets and slide the tamales out onto the platter. Serve the tamales with hot sauce.

The tamales can be prepared through Step 2 and refrigerated for up to 2 days.