To prepare cake, combine 1 cup boiling water and 2/3 cup cocoa in a bowl, stirring until smooth. Add 2 ounces bittersweet chocolate; stir until smooth. Cool to room temperature. Coat 2 (8-inch) round metal cake pans with cooking spray; line bottoms of pans with wax paper. Coat wax paper with cooking spray; dust each pan with 1 teaspoon flour.
Place 1 3/4 cups sugar, 6 tablespoons butter, and 1 teaspoon vanilla in a large bowl; beat with a mixer at medium speed 1 minute. Add egg whites, 1 at a time, beating well after each addition. Add sour cream; beat at medium speed 2 minutes. Weigh or lightly spoon 4 ounces flour (about 1 2/3 cups) into dry measuring cups; level with a knife. Combine 4 ounces flour, baking powder, baking soda, and 1/2 teaspoon salt in a bowl, stirring well. Add flour mixture and cocoa mixture alternately to sugar mixture, beginning and ending with flour mixture and beating just until combined.
Divide batter evenly between prepared pans. Bake at 350° for 30 minutes or until a wooden pick inserted into center comes out clean. Cool in pans 10 minutes on wire racks. Invert cake layers onto racks; cool completely. Discard wax paper.
To prepare filling, combine juice and next 6 ingredients (through 1/4 ounce bittersweet chocolate) in a small saucepan over low heat; bring mixture to a boil, stirring frequently. Cook 1 minute, stirring constantly. Pour into a bowl. Cover and chill. Uncover; fold in whipped topping.
To prepare glaze, combine milk, 1 tablespoon butter, and 4 ounces bittersweet chocolate in a medium microwave-safe bowl; microwave at HIGH for 1 minute, stirring every 15 seconds until smooth.
Place 1 cake layer on a plate. Spread filling over top, leaving a 1/4-inch border. Top with remaining cake layer. Spoon warm glaze over top of cake, allowing it to drip over the edges of cake.