1. Preheat grill to medium heat (300°). Spread raspberry preserves on 1 side of bread slices. Place 2 chocolate squares or half milk chocolate bar in center of 4 slices. Top with remaining bread slices. Spread about 2 teaspoons butter on outsides of each sandwich.
2. Grill sandwiches 2 minutes; carefully turn, and grill 2 more minutes or until bread is toasted and chocolate is soft. Sprinkle with sea salt, if desired. Let sandwiches stand 1 to 2 minutes, and cut in half.