Slice the walnut bread thin into 8 slices. Heat a large nonstick skillet or a griddle over moderate heat. Butter 1 side of each bread slice, then put 4 slices, buttered side down, on a work surface. Cut chocolate into chunks, thin enough to fit in a sandwich. Place on the bread, cover with another slice of walnut bread and butter the side facing up. Grill on both sides, about 3 to 5 minutes each. Serve warm.