Microwave chocolate and butter in large bowl, stirring often, until melted and smooth, 1 to 3 minutes; let cool slightly. In separate bowl, whisk flour, walnuts, baking powder, and salt together.
Slowly whisk granulated sugar into cooled chocolate mixture until combined. whisk in eggs, one at a time until combined. Fold in flour mixture until just combined. Cover and refrigerate until firm, about 2 hours.
Adjust oven racks to upper-middle and lower-middle positions and heat oven to 300 degrees. Meanwhile, line two rimmed baking sheets with parchment paper. Spread confections' sugar in shallow dish. Working with 1 tablespoon of dough at a time, roll dough into balls, then roll balls in confectioners' sugar to coat.
Transfer balls to prepared sheets, spaced about 2 inches apart. Bake cookies until edges are set but centers are still soft and puffy, 18 to 20 minutes, swiching and rotating baking sheets halfway through baking. Let cookies cool on sheets for 10 minutes, then transfer to wire rack and let cool completely before serving. Repeat with remaining dough using cooled, freshly lined baking sheets.
The recipe was alittle vague but after reading it a few times I took it to mean that you bake it 9-10 minutes on lower rack and then switch it to upper rack to finish baking for 9-10 minutes. It turned out well any way.