How over-carbed? You might just be able to uncap and recap...CO2 will come out of solution to re-fill that headspace. It'd be easy to try with just a couple bottles to see.
as for the cocoa, how young is the beer? cocoa will probably mellow with time, and stout is one fo those beers that only improves with some aging, at least in my humble opinion
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10