Substitute for dark malt extract?

So I was looking at this partial mash recipe for a smoked porter from BYO, and it calls for dark malt extract. I like to only work with light malt extract, but I'm not sure what the components of a dark extract are. Chocolate? Roasted? Dark crystal? What would you recommend to replace the dark DME with specialty grains in this recipe?

Additionally, I don't see any crystal type grains here. I was thinking about adding half a pound of something for body - thoughts?

to be honest you cant really be sure whats in any extract. if you go to the manufactures website they will get you a general idea of whats in there but noting to specific and never seem to give the exact ratios.

in my porters i use 0.5# Crystal Malt 60L, 0.5# Chocolate Malt, and 0.25# Black Patent Malt.

Dark extract is usually some mix of pale, crystal 60 (or higher), munich and black malts.

I would use 0.5-1.5 lbs of C80L. The recipe already has black and chocolate, so some crystal will do it.