Prepare pan: Remove sides from a 9-inch round springform pan (see note, below). Place a sheet of waxed paper over bottom, leaving an overhang; lock sides onto bottom, firmly securing paper. Spray inside of pan with cooking spray; line sides with a strip of waxed paper 28 inches long and 4 inches wide.
Make chocolate-ricotta mixture: Break 12 ounces of chocolate into pieces. Place in a heatproof medium bowl set over, not in, a pan of simmering water. Cook, stirring occasionally, until chocolate has melted, 8 to 10 minutes.
In a food processor, blend ricotta until very smooth, about 1 minute, scraping down sides of bowl. Add warm chocolate; blend until smooth. In a large bowl, beat cream until stiff peaks form. With a rubber spatula, gently fold in chocolate-ricotta mixture.
Assemble cake: Arrange half the cookies in an overlapping pattern to cover bottom of pan. Spoon half the chocolate-ricotta mixture on top of cookies; smooth top. Cover with remaining cookies; top with remaining chocolate-ricotta mixture, and smooth top. Cover with plastic wrap; refrigerate at least 6 hours and up to 2 days.
Before serving, release sides of pan and remove waxed paper from sides. Using bottom piece of waxed paper, pull cake onto a platter; with a metal spatula, lift cake and remove waxed paper. Using a vegetable peeler, shave remaining 2 ounces chocolate over top of cake. With a knife dipped in hot water, cut cake into 10 slices (wipe off knife, and dip in hot water after each slice).
The right pan: A springform pan has a clamp or spring that releases the sides of the pan from the bottom. It's ideal for cakes and tarts that would be hard to remove from a regular pan.