In a bowl, beat eggs, milk, sugar, vanilla and salt. Pour half into an ungreased 13-in. x 9-in. x 2-in. baking dish. Arrange six slices of bread in a single layer over egg mixture. Place one piece of chocolate in the center of each piece of bread. Top with remaining bread; pour remaining egg mixture over all. Let stand for 5 minutes. In a large nonstick skillet, melt butter over medium heat. Fry sandwiches until golden brown on both sides. Dust with confectioners' sugar. Cut sandwiches diagonally; serve warm.