Preheat oven to 180 C / Gas 4. Prepare and bake cake mix according to instructions for two 23cm round layers. Cool and remove from tins.
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 1/3 of the mousse, then cover with the remaining cake layer. Top with the remaining mousse (top and sides) and refrigerate until serving. Garnish with fresh fruit or chocolate shavings.