Adjust oven racks to the upper and lower middle positions and heat the oven to 425 degrees.
Working with one sheet at a time on a lightly floured work surface, trim the puff pastry sheets to 9- by 9-inch squares, then cut each sheet into nine 3-inch squares.
Working with one square of dough at a time, place a portion of chocolate onto the lower corner, brush two edges of the dough with beaten egg, and fold the dough in half diagonally to form a turnover. Place each turnover on a parchment-lined cookie sheet, spacing them about 2 inches apart (about 9 per sheet). Brush the dough tops with egg wash.
Bake until golden brown, switching and rotating cookie sheets halfway through baking to ensure even browning, about 15 minutes. Cool the turnovers briefly. Sprinkle with confectioners' sugar, if desired, and serve immediately.
Courtesy of Pam Anderson's How to Cook Without a Book. Published by Broadway Books, 2000.