Preheat oven to 175 degrees C (350 degrees F). Prepare and bake cake mix according to package directions for two 24cm (9inch) layers. Cool and remove from pans.
In a large bowl, mix together sweetened condensed milk and melted chocolate. Gradually stir in the water, then instant pudding until smooth. Chill for at least 30 minutes.
Remove the chocolate mixture from the refrigerator, and stir to loosen. Fold in whipped cream and return to the refrigerator for at least another hour.
Place one layer of cake onto a serving plate. Top with 1 1/2 cups of the mousse, then cover with the remaining cake layer. Frost with the remaining mousse, and refrigerate until serving. Garnish with fresh fruit or chocolate shavings.