Bake the chocolate cake according to the instructions on the box. Once removed from the oven, let it rest briefly in the pan, then transfer it to a rack to cool completely.

When the cake has cooled, carefully slice it horizontally into two even layers. Return the bottom half to the pan or place it on your serving plate.

In a large bowl, beat the softened cream cheese with the margarine and powdered sugar until the mixture is creamy and lump-free.

Fold the thawed whipped topping into the cream-cheese mixture gently until the filling is uniformly light and airy.

Spread the filling evenly over the bottom cake layer, then position the second layer on top, pressing lightly to level.

Stir the milk into the canned chocolate frosting until it reaches a pourable, spreadable texture. Smooth the frosting over the top of the cake.

Chill the assembled cake in the refrigerator for at least one hour to let the layers set. Slice and serve cold; store leftovers in the fridge.