Not *exactly* a flavor, but melanoiden. Either use melanoiden malt or aromatic if you cannot get the former. For a 5 gallon doppelbock maybe .5-.75 lbs.
Maybe even a pound would be OK, although I'd want to keep it somewhat balanced if you've got a bunch of crystal in there, or your FG will be way too high even if you mash at 145F.
Another way to develop that same flavor is to take about a gallon of your wort, and while you're heating up your kettle to boil, in a separate pot/burner, boil that gallon down until it's a quart or less. That will give you some killer malty melanoiden goodness.