Prepare and bake cake as directed on box. (2 round cake pans)
Filling: Mix and cream the butter and cream cheese. Add powdered sugar and fold in cool whip.
After cake is completely cooled split layers (TIP: you can place the uncut cakes on plates in a freezer for about 30 minutes to make a cleaner cut, then use UNwaxed dental to pull through the layer and make a wonderful even cut.) and spread filling on each layer and stack, do not spread filling all the way to the edge of the cake, the weight of the cake will press it out to the edge once you are finished, this keeps the filling from mixing with the icing
Now, if you have any filling left over you can mix it with the icing. It’s what we do. Then ice entire cake with the chocolate icing. You will be able to put a thick layer of filling between the layers.