With the rack in the middle position, preheat the oven to 275°F (140°C).
In a Dutch oven over high heat, brown the meat in the oil. Season with salt and pepper. Transfer to a plate and set aside.
In the same Dutch oven over medium heat, soften the vegetables in the rendered fat. Deglaze with the wine. Add the veal stock and the browned meat, and bring to a boil. Cover, transfer to the oven and braise until the meat is fork-tender, about 3 to 4 hours.
Remove the meat from the Dutch oven.
Whisk the cocoa powder into the sauce. Adjust the seasoning. Return the meat to the Dutch oven.