I am not an expert but my take on it is that you are really asking two things.
1) You want to do GOMAD but not GOMAD because you are actually only going to have two litres of milk a day instead of 4.
See
http://stronglifts.com/gomad-8-common-mistakes-you-must-avoid/ for some comment on 1/2 GOMAD or 1/4 GOMAD
2) You want to add in a high carb food (cocoa powder) to make up for not doing GOMAD.
Cocoa powder is not a substitute for milk. It has a totally different nutritional composition. I am not really qualified to comment on how apprpriate cocoa powder is going to be for your diet.
It may well be that adding cocoa powder to your milk will make the milk easier to drink and hence help you to stick to GOMAD, but the powder should not be substituted for the milik in gomad.
I guess what I am trying to say is you can go ahead and try 1/2 GOMAD + Cocoa powder and it may work for you, but you shouldn't confuse it with GOMAD and expect the same or similar results. That is something different.