To prepare cake, coat 2 (9-inch) round cake pans with cooking spray; line bottoms of pans with wax paper. Lightly coat wax paper with cooking spray; dust pans with 1 tablespoon flour.
Combine cocoa, 1/2 cup boiling water, and 1 ounce bittersweet chocolate in a small bowl, stirring until smooth; cool.
Place 3/4 cup granulated sugar, 1/2 cup dark brown sugar, and 1/4 cup butter in a large bowl; beat with a mixer at medium speed 2 minutes or until well blended.
Add egg substitute; beat well.
Lightly spoon 2 cups flour into dry measuring cups; level with a knife.
Combine flour, baking soda, and salt, stirring well with a whisk.
Combine buttermilk, sour cream, and 2 teaspoons vanilla, stirring well.
Add flour mixture and buttermilk mixture alternately to sugar mixture, beginning and ending with flour mixture. Stir in cocoa mixture.
Spoon batter into prepared pans. Sharply tap pans once on counter to remove air bubbles.
Bake at 350 for 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pans 10 minutes; remove from pans.
Remove wax paper; discard. Cool cakes completely on wire racks.
To prepare filling, place chips in a microwave-safe bowl, and microwave at high for 1 minute, stirring every 15 seconds. Cool slightly. Fold 1/4 cup whipped topping into melted chips. Repeat procedure 7 times with remaining whipped topping. Chill 1 hour.
To prepare ganache, combine 2/3 cup granulated sugar and next 4 ingredients (through 2 ounces bittersweet chocolate) in a medium saucepan over medium heat; bring to a boil. Cook 2 minutes or until sugar dissolves and chocolate melts, stirring constantly.
Remove from heat. Stir in 1 teaspoon vanilla; cool completely. Chill 30 minutes or until thick.
Place 1 cake layer on a plate; top with filling, spreading out to edges.
Place remaining cake layer on filling.
Spread ganache evenly over top of cake; let drip down sides. Chill 20 minutes or until set.
