I just literally finished bottling a Chocolate Raspberry Chambourcin that I started in early October, and I would also like to make a chocolate mulberry (need to find fresh mulberries). I would not hesitate to use the basis of the recipe that I used for my CRC, which I have compared in blinded side by side with 30 people against the Chocolate Raspberry Port kit--and 3 preferred the kit.
I did not use nibs, and have always used cocoa powder. I have used Hershey's basic plain unsweetened and their Special Dark cocoa powder. OR, I would recommend using DUTCH processed cocoa. The nibs will take so very long to impart the flavor you are seeking, THOUGH when you start this wine you could put your nibs in the cognac or brandy--whatever you plan to fortify with, that way you will have a nib infused liqueur for fortification.
Here is the recipe, substituted mulberries, and I broke it down PER GALLON:
5# fresh mulberries
1-2 cans Welch's grape concentrate,frozen but thawed
4.8 oz BY WEIGHT of Hershey's Cocoa Special Dark cocoa powder (or use a DUTCH processed unsweetened cocoa powder)
water to 1 gallon + 3 cups
Invert sugar to desired OG (I went with 1.090)
Pectic enzyme, yeast nutrient/energizer and k-meta per standard winemaking process
**likely will not need acid since the chocolate tends to provide that and the mulberries should provide enough tannin**
I found it easiest to combine the cocoa with hot water and make a slurry, then add the fruit component in a straining bag. Allow all this to rest overnight, add the water to 1 gallon, stir well and check SG. Then start adding invert sugar syrup. Add final amount of water if necessary to reach the +3 cup mark. This will foam like crazy due to the chocolate as it ferments and it is common to have a 2-3" chocolate cap.
And then of course you can feed additional sugar in increments to max out the yeast's alcohol potential or fortify on the backend, or both.
If it were me, I would make at least a 3 gallon batch. I made a 10 gallon batch of Chocolate Chambourcin Raspberry and after all racking ended up with 8 gallons of wine. Quite a bit of chocolate sludge, but then again I did start with 10 gallons.
And I will definitely make this again, once harvest time is upon us. I did backsweeten and ended up at 1.018, higher than I typically prefer, but for the that particular wine it was the preferred amount during trials. I ended up finishing 5 gallons as 'wine' and 3 gallons 'port-style' with my nib-infused brandy.
I would like to thank Col Wade of WinePressUS forum for his Chocolate Raspberry Chambourcin recipe and permission to share with anyone that would like to make their own. It is easily adaptable.
Please let us know what recipe you do opt to use.