Pull liver apart, break each part and remove any skins or cores, being careful to keep as much intact as possible. Knead liver softly, season with salt and pepper and baste in a bowl with port wine and cognac. Mix gently and leave covered overnight in a cool place.
Spread liver pieces carefully fill into the mold, making sure that no gaps arise. Smooth surface and press down. Cover mold with a lid and place in a pot of hot water exactly 80°C (approximately 175°F) using a thermometer, and cook in the oven with a temperature of 140°C (approximately 275°F) for 35-40 minutes. Allow finished terrine to rest for 24 hours in a cool place. To serve, cut into slices and garnish as desired with grapes.