Heat a grill pan or a large heavy nonstick skillet over medium-low heat. Meanwhile, brush one side of each bread slice with some of the melted butter. Place half of the bread slices on a work surface, buttered side down; sprinkle with chocolate, covering the bread to within 1/4-inch of the crust. Top with remaining slices, buttered sides up. Place sandwiches, 2 at a time, in grill pan; weigh down top of sandwiches with a large heavy skillet.

Grill the sandwiches over medium-low heat for 6 to 8 minutes or until the chocolate is melted and bread is golden brown, turning once halfway through grilling time. Repeat with remaining sandwiches. (Or preheat a covered indoor grill. Place sandwiches, two at a time, in preheated grill, cover, and grill 4 minutes or until chocolate melts and bread is golden.)

To serve, cut each sandwich into quarters. Dust with powdered sugar. Serve warm. Makes 4 servings.