Here's a recipe I've made and done aged on cacao nibs before. It turned out quite nicely (scaled back from 5 gallons down to 3 gallons). I've just brewed this again (using Wyeast's Scottish ale yeast in combo with US-05), but will be ageing it on oak and whiskey not chocolate this time. Anyway, the special b gives it a really nice character that complements chocolate very well.
Amount Item Type % or IBU
1.80 lb Light Dry Extract (8.0 SRM) Dry Extract 21.43 %
1.80 lb Light Dry Extract (8.0 SRM) Dry Extract 21.43 % (late addition: 15 min boil)
1.80 lb Pale Malt (2 Row) US (2.0 SRM) Grain 21.43 %
1.20 lb Barley, Flaked (1.7 SRM) Grain 14.29 %
0.60 lb Special B Malt (120.0 SRM) Grain 7.14 %
0.45 lb Black (Patent) Malt (500.0 SRM) Grain 5.36 %
0.45 lb Chocolate Malt (350.0 SRM) Grain 5.36 %
0.30 lb Roasted Barley (500.0 SRM) Grain 3.57 %
0.67 oz Warrior [15.40 %] (60 min) Hops 58.7 IBU
0.67 oz Willamette [4.90 %] (10 min) Hops 6.8 IBU
4.0 oz Cacao Nibs, Whiskey Soaked (Secondary 14.0 days) Misc
1 Pkgs US-05 Yeast-Ale
70% efficiency will get you about 1.088 for the OG and 65 IBUs or so.
Check out the stout section in the recipe database, and if you see a imperial stout that looks good, add a few ounces of cocoa powder to the end of the boil or nibs in the secondary. There are plenty of resources on here about using chocolate in brewing.
