1. Preheat oven to 325°. Line a lightly greased 15- x 10-inch jelly-roll pan with parchment paper; lightly grease parchment paper.
2. Microwave chocolate and 1/4 cup water in a large microwave-safe bowl at HIGH 1 minute or until melted and smooth, stirring at 30-second intervals. (Do not overheat mixture.)
3. Beat egg yolks and 3/4 cup sugar at high speed with an electric mixer 3 minutes or until mixture is thick and pale yellow. Whisk into chocolate mixture.
4. Beat egg whites at high speed with an electric mixer until stiff peaks form; fold into chocolate mixture.
5. Bake at 325° for 15 to 18 minutes or until puffed. Remove from oven; place a damp kitchen towel over cake in pan, and cool completely on a wire rack (about 30 minutes).
6. Remove damp towel. Place a 20- x 15-inch piece of parchment paper on work surface. Dust cake with cocoa, and invert onto parchment paper. Carefully peel parchment paper from top of cake.
7. Beat cream, orange liqueur, and remaining 1/4 cup sugar at high speed with an electric mixer until stiff peaks form. Spread cream mixture over top of cake, leaving a 1-inch border on all sides. Lift and tilt parchment paper at 1 short side, and carefully roll up cake, jelly-roll fashion, using parchment paper to lift and roll cake. Wrap rolled cake in parchment paper, and place on baking sheet. Chill 2 to 8 hours. Transfer cake to a serving platter; remove and discard parchment paper. Garnish cake, if desired.