Step 1
Preheat oven to 350 degrees F (175 degrees C). Grease and flour two 9-inch round baking pans.
Step 2
Mix cake mix 1 cup water oil and eggs in a large bow until well blended. Pour batter into prepared baking pans.
Step 3
Bake in the preheated oven until a toothpick inserted into the center comes out clean 26 to 31 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack. Slice one cakes in half horizontally and place one half in a 10-inch springform pan. Save remaining 3/4 cake for another use.
Step 4
Melt chopped baking chocolate and cream cheese in a large microwave-safe bowl in 30-second intervals stirring after each melting for 1 to 3 minutes (depending on your microwave). Set aside to cool.
Step 5
Mix 1 teaspoon gelatin with 1 tablespoon cold water in a small bowl for 1 minutes; add boiling water and stir until gelatin is dissolved. Allow to cool.
Step 6
Beat 3/4 cups heavy whipping cream until foamy in a large bowl. Add 1/2 teaspoon sugar and gelatin mixture; continuing to beat until stiff peaks form. Lift your beater or whisk straight up: the whipped cream should form a sharp peak that holds its shape. Fold about 1/4 the whipped cream into the cooled chocolate cream cheese mixture until well blended; fold remaining whipped cream into chocolate mixture to create a mousse. Spread mousse over cake arranged in springform pan. Refrigerate while preparing next layer.
Step 7
Beat egg yolks in a large bowl until yolks are a pale yellow. Beat in 1/4 cup sugar 3 tablespoons flour and 1 teaspoon gelatin until mixed.
Step 8
Bring 1 3/4 cups heavy whipping cream and vanilla extract to a boil in a saucepan; lower heat to medium. Pour about 1/2 cup hot cream into egg mixture while stirring; stir remaining cream mixture into bowl. Pour cream mixture through a strainer into the saucepan; continue cooking and stirring until mixture is thick enough to coat the back of a spoon about 10 minutes. Allow custard to cool then pour over mousse layer in springform pan. Place cake in freezer while making remaining layer.
Step 9
Reserve 1/2 cup chocolate chips; pour remaining chocolate chips in a large bowl.
Step 9
Bring 1 1/2 cups heavy whipping cream and butter to a boil in a clean saucepan. Pour cream mixture into the bowl over chocolate chips. Stir until smooth and chocolate is melted. Allow mixture to cool until chocolate ganache begins to thicken.