In a food processor, puree 2 15-ounce containers ricotta with 12 ounces melted and cooled semisweet chocolate until very smooth.
Line an 8½-by-4½-inch loaf pan with parchment paper, leaving an overhang on 2 sides. Layer the ricotta mixture and half of a 9-ounce package of chocolate wafer cookies in the prepared pan (3 layers of the ricotta mixture and 2 layers of the cookies, beginning and ending with the ricotta mixture).
Refrigerate for at least 12 hours and up to 2 days. To serve, remove from pan, slice, and sprinkle with ¼ cup shaved semisweet chocolate.