Preheat oven to 350 degrees. Grease and flour two nine inch cake pans...Prepare cake mix following instructions on box. Cool in pans for a few minutes before removing and adding to baking racks. Cook completely.
In a large bowl, mix together the condensed milk and the melted chocolate. Gradually stir in the water and then the pudding mix until well blended. Refrigerate for at least 30 minutes.
Remove mixture from refrigerate and stir to loosen up. Fold in whipped cream and chill for another hour.
Place one layer of cake onto serving plate. Top with one and a half cups of the mousse. Place top layer over filling and ice top and sides with remaining mousse. Refrigerate. Refrigerate until ready to serve. This cake is really pretty topped with shaved chocolate.