Here is the grist for Youngs double chocolate stout, this is from young s head brewer himself
"The grist is made up with about 3% Roasted
Barely, 3% chocolate malt, 8% oats, 9% dark fermenting syrup (of high molasses content) and 77% Pale Ale malt. (figures by extract).We then add a small amount of chocolate to the boil (we tend to use plain-high cocoa solids varieties). After fermentation and filtration we add a small amount of chocolate essence
to give aroma.
As I mentioned earlier, re-producing this beer on a domestic scale would be very difficult."
I suspect that the syrup and essence maybe trade only, but experimentation could yield good results