Heat the duck fat to 50°C and then submerge the whole foie gras lobe in the fat for about 30 minutes, turning it every ten minutes, until it reaches 50°C at its centre. Drain the foie gras on a grill, leave it to cool a little and then remove the nerves with the back of a spoon. Season the lobe with salt and pepper and then roll it in the cocoa and make a sausage shape, placing the ginger in the centre in some cling film. Keep chilled. Serve after forty-eight hours at the earliest.