1. Preheat oven to 350°.
2. To prepare cake, combine 1 cup boiling water and 1/2 cup cocoa. 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 pans with remaining 1 tablespoon cocoa.
3. Place 1 3/4 cups granulated 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 for 2 minutes. Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, baking soda, and 1/2 teaspoon salt in a bowl, stirring with a whisk. Add flour mixture and cocoa mixture alternately to sugar mixture, beginning and ending with flour mixture; beat just until combined.
4. Divide batter evenly between the prepared pans. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out with moist crumbs clinging. Cool 10 minutes in pans on wire racks. Remove from pans; cool on wire racks. Discard wax paper.
5. To prepare filling, combine 1/3 cup milk and the next 3 ingredients (through dash of salt) in a saucepan over medium-low heat; bring to a boil, stirring constantly. Cook 1 minute or until thick, stirring constantly. Remove from heat. Add milk chocolate, stirring until smooth. Pour into bowl. Cover and chill. Uncover; fold in whipped topping.
6. To prepare glaze, combine powdered sugar and remaining ingredients in a saucepan over low heat. Cook for 2 minutes, stirring frequently. Place 1 cake layer on a plate. Spread filling over cake, leaving a 1/4-inch border. Top with remaining layer. Drizzle glaze over top of cake, spreading it out over edges.