Set oven rack about 6 inches from the heat source and preheat the oven's broiler. Place bananas on a baking sheet.
Broil bananas until skins are black all over, about 8 minutes. Remove from oven. Split skins lengthwise and place chocolate chunks inside the open skins. Broil until chocolate is melted, 2 to 3 minutes.
Serve creme fraiche alongside bananas. Sprinkle grated chocolate over the creme fraiche.