Soften gelatin in water in heat-proof dish, 3 minutes. Place dish in skillet of simmering water; stir gelatin to dissolve.
Slice cake into scant 1/2-inch-thick slices. Fit slices in bottom of 13 x 9 x 2-inch baking pan in single layer, cutting and adding scraps to fit snugly. Drizzle liqueur over cake.
Process cream cheese, ricotta, sugar and cocoa in food processor until smooth. Add gelatin mixture, processing to blend. Add chocolate pieces; process just to combine. Scrape into prepared pan, spreading over cake. Cover with plastic wrap. Chill 3 hours or until set.
Dust with cocoa; cut in squares. Makes 15 squares.