In a saucepan, heat margarine, milk, cocoa powder and vanilla extract and stir
briskly with a wire whisk until the mixture comes to a boil. Remove from the heat
and stir in the confectioners' sugar. Stir with a wire whisk for about one minute,
until the sauce is fairly smooth. Place the cake on a cake plate that has a cover.
With a long, sharp serrated knife, slice the cake into 3 equal horizontal layers.
Gently remove the top 2 layers and set aside.
Spread 1 2/3 containers of pudding on the bottom layer (over the section where
it was cut) to form a filling. Top with the second cake layer and spread on the
remaining pudding. Top with the remaining layer. Pour the chocolate sauce in a thin
stream directly on top of the cake. Allow the sauce to run down the sides, covering
the cake almost entirely. There will be quite a bit of sauce on the plate. Cover
and refrigerate until ready to serve.
To serve, cut into slices with a sharp, serrated knife, using a sawing motion.
Serve extra sauce from the plate along with the slices, if desired.