Bake the cake according to the package directions in 2 8-inch round pans. Cool completely.

Make the mousse: Combine the gelatin and cold water in a small bowl and allow to sit for 1 minute. Whisk the boiling water into the gelatin mixture, then allow to cool.

In a medium bowl, combine the sugar, cocoa powder, cream and vanilla. Beat until the mixture forms medium-stiff peaks. Beat in the gelatin mixture. Refrigerate for 30 minutes.

Make the ganache: Combine the chocolate chips and cream in a microwave safe bowl. Heat on high for 3-4 minutes, then stir until smooth. (If the ganache is too thin, refrigerate until it thickens to your desired thickness.)

To assemble the cake: Using a serrated knife, level the domes off of the cooled cakes. Place one of the cakes on a serving plate. Spread the mousse over the top of the cake. Sprinkle with Heath bits. Place the second cake on top. Spoon the ganache over the top of the cake, allowing it to drip over the sides. Sprinkle with additional Heath pieces. Refrigerate until ready to serve.