Give it time. I made a RIS back in early March which I just bottled recently. Lots of Roasted barley, chocolate rye, and (regular) chocolate malt. Hydro samples tasted awful before and right after primary fermentation, but by the time I bottled it a couple of weeks ago, it was starting to get very balanced and tasty. I've always noticed that a couple months of aging will take these beers with lots of dark, roasted malts from acrid messes to smooth, chocolaty treats almost without fail.
Also of note, this grainbill looks much more like a stout than anything else. It actually looks quite good, but I wouldn't call it a brown ale. Just think of it as a stout and you'll love it.