Lots of the HBS's sell
alcohol boost packs like this. For $2.00, you can safely add about 1% ABV without affecting flavor. Otherwise, just use alot of fermentable malts. Generic light extract added to the boil will help. I assume you're trying to make an imperial, given your alcohol %. Here's my recipe. In my case, I went a little low on the IBU's because I didn't want the hops to clash with the chocolate and coffee. In your case, I'd let the IBU's stay between 50 and 100 for this style. Max SG is 1.114 according to BJCP. Obviously, omit the chocolate and coffee unless you want to make this beer (which is freakin amazing, BTW).
If I were you, I'd consult the BJCP guideline book and read up on russian imperial stout while you're at it.
Imperial Chocolate-Espresso Stout
A ProMash Recipe Report
BJCP Style and Style Guidelines
-------------------------------
13-F Stout, Russian Imperial Stout
Min OG: 1.075 Max OG: 1.114
Min IBU: 50 Max IBU: 100
Min Clr: 30 Max Clr: 48 Color in SRM, Lovibond
Recipe Specifics
----------------
Batch Size (Gal): 5.00 Wort Size (Gal): 5.00
Total Extract (Lbs): 13.00
Anticipated OG: 1.092 Plato: 21.92
Anticipated SRM: 51.4
Anticipated IBU: 65.8
Wort Boil Time: 60 Minutes
Pre-Boil Amounts
----------------
Evaporation Rate: 15.00 Percent Per Hour
Pre-Boil Wort Size: 5.88 Gal
Pre-Boil Gravity: 1.078 SG 18.85 Plato
Formulas Used
-------------
Brewhouse Efficiency and Predicted Gravity based on Method #1, Potential Used.
Final Gravity Calculation Based on Points.
Hard Value of Sucrose applied. Value for recipe: 46.2100 ppppg
% Yield Type used in Gravity Prediction: Fine Grind Dry Basis.
Color Formula Used: Morey
Hop IBU Formula Used: Rager
Additional Utilization Used For Plug Hops: 2 %
Additional Utilization Used For Pellet Hops: 10 %
Grain/Extract/Sugar
% Amount Name Origin Potential SRM
-----------------------------------------------------------------------------
3.8 0.50 lbs. Chocolate Malt America 1.029 350
7.7 1.00 lbs. Roasted Barley America 1.028 450
3.8 0.50 lbs. Crystal 10L America 1.035 10
61.5 8.00 lbs. Briess LME- Dark America 1.035 23
15.4 2.00 lbs. Generic DME - Light Generic 1.046 8
3.8 0.50 lbs. Brown Sugar Generic 1.046 4
3.8 0.50 lbs. Molasses Generic 1.036 80
Potential represented as SG per pound per gallon.
Hops
Amount Name Form Alpha IBU Boil Time
-----------------------------------------------------------------------------
0.50 oz. Pacific Gem Whole 13.60 25.2 60 min.
1.00 oz. Northern Brewer Pellet 6.50 26.5 60 min.
0.50 oz. Cascade Whole 5.75 10.7 60 min.
0.50 oz. Saazer Pellet 3.30 3.4 30 min.
Extras
Amount Name Type Time
--------------------------------------------------------------------------
2.00 Tsp Gypsum Powder Other 60 Min.(boil)
8.00 Oz Unsweetened Baker's Chocolate Other 20 Min.(boil)
0.25 Tsp Irish Moss Fining 15 Min.(boil)
12.00 Cup(s) Brewed Drip Coffee Coffee 21 Days(fermenter)
12.00 Oz Brewed Espresso Coffee 0 Min.(fermenter)
Yeast
-----
DCL Yeast US-56 Safeale American Ale Yeast