Roll out 1 puff pastry sheet into a 10-inch square; cut into 2 1/2-by-5-inch rectangles (8 total). Sandwich between 2 unlined rimmed baking sheets and bake at 400 degrees F until golden, 30 minutes. Brush with melted butter and sprinkle with granulated sugar. Cool. Cook 1/3 cup milk, 2 tablespoons sugar and 1 tablespoon cornstarch in a small skillet over medium heat, whisking, until thick. Stir in 2 ounces chopped semisweet chocolate; cool. Fold in 1 1/2 cups whipped cream. To assemble, layer 2 pastry rectangles with the chocolate cream and raspberries. Dust with confectioners' sugar.