Preheat oven to 350°.
To prepare cake, place juice and dates in a blender; process until smooth. Combine juice mixture, cocoa, and 2 tablespoons sugar in a medium saucepan over medium heat; bring to a simmer. Cook 2 minutes, stirring constantly until sugar dissolves. Remove from heat, and place in a large bowl. (Mixture will be thick.)
Beat vinegar and egg whites with a mixer at high speed until soft peaks form. Add 2/3 cup sugar, 2 tablespoons at a time, beating until stiff peaks form.
Add egg yolks to cocoa mixture; stir until smooth. Gently stir one-fourth of egg white mixture into egg yolk mixture; gently fold in remaining egg white mixture.
Pour batter into a 9-inch springform pan coated with cooking spray. Bake at 350° for 30 minutes or until firm in center. (Cake will rise over rim of pan and collapse as it cools.)
To prepare candied orange rind, combine 3/4 cup water and 1/2 cup sugar in a small saucepan; bring to a boil, stirring occasionally until sugar dissolves. Add rind to pan. Reduce heat, and simmer 20 minutes, stirring occasionally. Using a fork, remove rind from pan; dredge in remaining 2 tablespoons sugar. Let rind stand 30 minutes or until dry. Garnish center of cake with candied rind.