If you're looking for a strong one (think ~10%), you could add chocolate to this recipe, I'm supposing you mean cocoa powder or something like pure chocolate extract? (as this recipe already involves chocolate malt)
If you were to add chocolate to this I'd suggest scaling the recipe and adding in some lactose and a cup or so of cocoa to the boil.
You can also add a drop or two of pure chocolate extract to each at bottling to really make that chocolaty aroma pop.
If you're looking for a more mild beer and really want to focus on the chocolate, I'd recommend this recipe by Barleywater
, I made some modifications and also added fresh picked blackberry as a blackberry chocolate stout, great recipe.
Let me know what you decide!