Grease (2) 13 x 9-inch cake pans, and preheat oven to 350 degrees F.

In bowl of stand mixer, mix cake according to package directions.

Divide batter evenly between prepared cake pans, and bake according to package directions. Allow to cool on wire racks for 10 minutes. Remove from pans, and cool completely.

While cake is baking prepare ganache so that it can cool along with the cake.

Then cut a piece of cardboard to 13 x 9-inches, and cover it with foil. Set aside.

On high power, heat 1¼ cups heavy cream in the microwave for 1-2 minutes, until very hot and simmering, but not boiling.

While cream is heating, break chocolate into pieces, and put in a mixing bowl.

Pour hot cream over chocolate in bowl, and allow to stand for 10 minutes. Then stir until all pieces are melted and ganache is smooth. Allow to cool to room temperature, stirring occasionally.

Once the cake is completely cool, combine 2 cups heavy cream, pudding mix, powdered sugar, and vanilla, and beat for two minutes (or until stiff peaks form), frequently scraping sides of bowl.

Place one completely cooled cake on the prepared foil-lined cardboard. Immediately spread the cream filling over it and smooth. Top with second cake. Using an offset spatula, smooth the edges.

Refrigerate entire cake at least 2 hours or, preferably, overnight, and serve cake cold.