Roasty, save the Light DME for another style - using it would not result in a stout if I'm not mistaken.
I'm thinking of making this recipe with some minor variations. I do have questions though - what do you guys think of adding an additional 1 lb. of dark LME? Is 8 oz. of lactose enough to qualify this as a milk/sweet stout recipe? What is the advantage to use cocoa nibs over cocoa powder (if any)?