preheat the oven to 400°. Line a baking sheet with parchment paper. Melt the butter in a small pan over low heat. Add the chocolate and stir until smooth. Set aside to cool. Dust a work surface with powdered sugar and place the pastry on top. Roll out into a 12 x 10" rectangle. Trim the pastry so that the edges are straight
Fold the pastry in half lengthwise to mark the center and unfold again, leaving a mark down the middle. When the chocolate mixture is thick and cool, pour it onto the pastry, working quickly so that it doesn't set. Use a spatula to spread the chocolate evenly over the pastry. It is important that the chocolate mix is not too thick as it will run out during cooking. Sift the cocoa over the top.
Begin to roll the sides of the pastry rectangle to the middle, where the mark is. Repeat for the other end, until it meets the already rolled half. Roll carefully in the remaining powdered sugar and transfer to a plate. Refrigerate for 15 minutes. When firm, cut across the rolls in 1/4 inch thick slices. Place on the prepared baking sheet, spacing them 1 inch apart. Bake until golden, 8 to 10 minutes. Place on wire racks and let cool completely. Serves 8 to 10