For the best "crackle" effect, reroll balls in powdered sugar just before baking to cover any exposed dough.
Microwave first 3 ingredients in a medium-size microwave-safe bowl at HIGH 1 to 2 minutes or until butter and chocolate are melted and smooth, stirring every 30 seconds. Stir in granulated sugar and next 2 ingredients until blended. Stir together flour, cocoa, and baking powder in a small bowl. Stir flour mixture into chocolate mixture until well blended. Cover bowl with plastic wrap, and chill 1 hour to 5 days.
Preheat oven to 350°. Carefully drop dough by level spoonfuls into powdered sugar, using a 1 1/2-inch cookie scoop, and roll to coat. Place coated cookies 1 inch apart on 2 parchment paper-lined baking sheets.
Bake, in batches, at 350° for 10 minutes or until cookies are crackled. Cool completely on a wire rack (about 20 minutes).