Position racks in the top and bottom thirds of the oven; preheat to 300 degrees . Place the chocolate and butter in a small bowl. Microwave on high, stirring every 30 seconds, until melted; let cool. Scrape into a large bowl. Add the flour, sugar, walnuts, eggs, cocoa powder, baking powder and salt. Using an electric mixer on medium, beat until well blended, stopping occasionally to scrape down the sides of the bowl, about 1 minute.
Using a 1-inch (2 tsp.) cookie scoop, scoop the dough and roll in the confectioners' sugar. Divide between two nonstick or parchment-lined baking sheets, spacing about 2 inches apart. Bake until crackly on top and soft in the middle, rotating the sheets and reversing from the top to bottom racks halfway through baking, about 15 minutes. Transfer the cookies to a wire rack; let cool.