Preheat oven to 450 degree F and butter a large baking sheet. Cut the chocolate into eighteen 2-by 1/2-inch pieces.
Carefully unfold pastry sheet and roll out on a lightly floured surface with a floured rolling pin to form a 12-1/2-inch square. Trim edges of pastry with a pizza cutter or sharp knife to form a 12-inch square. Cut pastry into three 4-inch strips. Cut strips crosswise to make eighteen 2- by 4-inch rectangles.
Working with 1 rectangles at a time, brush top with egg wash and place 1 piece of chocolate 1 inch from short end of rectangle and roll up pastry over chocolate to encase chocolate and form a log. Press edge of pastry to seal, leaving ends of log open, and arrange pastry seam sides down on baking sheet. Brush tops of pastry with some of egg wash.
Repeat with remaining pastry and chocolate in same manner, and freeze pastry on baking sheet until firm, about 10 minutes.
Now, bake pastries in middle of oven until puffed and golden brown on top, about 10 minutes (don't worry if some chocolate leaks out). Transfer with a metal spatula to wire racks to cool. Dust tops of pastries with confectioners' sugar.