1. Combine crumbs and melted butter. Pat evenly over bottom and about 1/2 inch up side of a removable-rim 9-inch cheesecake or cake pan (at least 1 3/4 in. deep).
2. Bake in a 350° oven until crust is slightly toasted, 10 to 12 minutes.
3. Meanwhile, in a blender or food processor, whirl cottage cheese, cream cheese, eggs, and egg whites until very smooth.
4. In a large bowl, mix sugar and cocoa. Add cheese mixture, liqueur, and orange peel.
5. Heat chocolate in a microwave-safe bowl in a microwave oven at full power in 5-second intervals until soft. Stir until smooth, then scrape into cheese mixture. Beat with a mixer until smoothly blended. Pour into hot or cool crust.
6. Bake in a 350° oven until center barely jiggles when cake is gently shaken, 40 to 45 minutes. Run a thin-bladed knife between cake and pan rim. Refrigerate cake, uncovered, until cool, at least 2 1/2 hours. Serve, or if making ahead, wrap airtight when cool and chill up to 2 days.
7. Remove pan rim and cut cake into wedges.