Make the cake mix according to the manufacturer's recipe. Pour the batter into a greased and floured sheet pan. Bake until done, times will vary based on oven and type of pan used.
When the cake has cooled, cut into 4 inch wide strips, trimming the tops of the cake to create a flat surface for a uniform cake.
Follow the instructions on the package to make Mousse
Place a slice of the cake on a fondant screen and coat with a 1/2 inch thick layer of mousse. Add a second layer, and coat with a 1/2 inch layer of mousse, repeat with an additional layer and top with a final slice.
With an icing spatula, use additional mousse to fill in any voids or cracks in the sides of the cake to create a smooth surface and flat appearance.
Assemble Ganache - Place the chocolate chips in a large bowl. Heat the heavy cream over medium heat to just before boiling. Pour cream over chocolate chips and stir with a wire whisk until the chocolate is fully dissolved and is a dark, shiny mixture.
Take the hot ganache and slowly pour over the top of the cake to cover. Using a spatula smooth the surface, and pour additional Ganache allowing it to flow down the sides smoothing it with a spatula.
Place additional mousse in a pastry bag and use it to pipe flourishes and decorations to enhance the appearance. Refrigerate until serving.
Optional: when layering the mouse, place cleaned raspberries, sliced strawberries or cherries to add a twist to the flavor