For crust: Position rack in center of oven and preheat to 350 degrees F; place cookies, walnuts, sugar and chocolate in processor; process until finely chopped; add butter and process until moist crumbs form; transfer crumbs to a 10 inch springform pan; Press on bottom and 2 inches up sides of pan; set on cookie sheet and bake for 10 minutes; transfer to rack and cool; maintain oven temperature.
For filling: Melt unsweetened chocolate, stir until smooth; cool slightly; beat cream cheese with 1/2 cup sugar in large bowl until fluffy; add eggs, one at a time, beating until just incorporated; transfer 2 1/2 cups plain batter to a medium-sized bowl and set aside; add brown sugar to batter remaining in large bowl and beat until combined; mix corn syrup, then melted chocolate into this batter; pour 2/3 cup of this batter into a small bowl and set aside; pour remainder of batter in big bowl into crust; set on cookie sheet and bake until filling is barely set, about 15 minutes; cool 5 minutes; scald cream with orange and lemon zests in small saucepan; remove from heat; add white chocolate and stir until melted and smooth; cool slightly; pour white chocolate mixture to reserved 2 1/2 cups batter in medium bowl and stir; mix in liqueur; starting at outside edge, carefully spoon white batter over chocolate layer; set on cookie sheet and bake until sides are puffed and center moves only slightly when shaken, 35-40 minutes; cool 5 minutes; spoon reserved chocolate batter in the small bowl by tablespoonfuls into the center of white chocolate layer; using back of spoon, spread evenly to within 1/2 inch of cake edge; set on cookie sheet and bake 10 minutes; transfer to rack to cool; using small, sharp knife, cur around edge of pan sides to loosen; cool cheesecake completely in pan; ccover and refrigerate when completely cooled (prepare 2 days before you plan to serve this).
For dark and white chocolate leaves: Line 2 cookie sheets with foil; place semisweet chocolate and 1/2 tsp Crisco in small bowl; melt; using small metal (offset works best) spatula or back of spoon, spread thin layer of chocolate over veined underside of 6 leaves, being careful not to drip any chocolate onto other side of leaf; [lace on 1 cookie sheet; chill until set, about 5 minutes; spread second, thin layer of chocolate on each leaf, remelting chocolate if necessary; refrigerate 30 minutes.
Do the same with white chocolate.
Starting at stem end, gently peel leaves off, leaving only chocolate imprint (can be prepared 1 day ahead; refrigerate in single layers in container).
To make orange threads: Using vegetable peeler, remove orange peel in large strips; cut strips into fine, long julienne; blanch 30 seconds; drain well; dry thoroughly on paper towels (can be prepared 1 day ahead).
To make whipped cream topping: Beat cream and sugar until soft peaks form; add liqueur and beat until stiff peaks form.
To assemble this masterpiece: Release sides of pan from cheesecake; spoon whipped cream into pastry bag fitted with large star tip; pipe decorative border around edge of cake; set alternating white and dark chocolate leaves at a slight angle (with 1 edge barely inserted into cream) atop cream border and into dark part of cake; arrange orange threads decoratively on cake, curling them by giving them a couple of twists.
Be brave enough to actually cut into this beauty cause it tastes delicious as well as being so gorgeous!