I would not substitute chocolate for the corn sugar in your porter.
If I were to make this according to the recipe, I would sub in about 1lb extra light DME, and use a tincture of cocoa nibs into the keg or bottles (4oz vodka on 2oz cocoa nibs for 1wk, for 5 gallons).
If I were making a Chocolate porter without the recipe, I would go something like this:
7lb Extra Light DME
.5lb Medium (60-80L) Crystal
.5lb Chocolate malt
.5lb Black (patent) malt
2oz (meh, I think this might be overzealous, 1oz or about 25-30 IBU) galena (or magnum) for Bittering
NO Flavor hops - (I think they would clash with the chocolate)
and use American ale, or some British strain.
Add Chocolate tincture (as prepared above) to bottling bucket or keg.
if you want to meet a specific flavor standard, you can take a dropper of some sort, and add a measured amount of Chocolate tincture to 1 pint of beer, and scale up to the full amount. I personally have found that 4oz tincture to 5gal beer is exactly where I like it.
As always, YMMV