These fudgy, crinkly cookies are perfect for chocolate lovers.
Adjust oven rack to middle position and heat oven to 325 degrees. Line rimmed baking sheet with parchment paper.
In medium bowl, whisk together flour, cocoa, baking powder, baking soda, and salt. In large bowl, whisk brown sugar, egg and egg yolk, and vanilla until combined.
Place chocolate in large zipper-lock plastic bag and seal, removing as much air as possible from bag. Use rolling pin to gently pound chocolate into small pieces.
In small microwave-safe bowl, combine pounded chocolate and butter. Heat in microwave at 50 percent power until melted, 1 to 2 minutes. Use oven mitts to remove bowl from microwave. Use rubber spatula to stir chocolate mixture until well combined and shiny.
Add chocolate mixture to brown sugar mixture and use rubber spatula to stir until combined. Stir in flour mixture until no dry flour is visible. Let dough sit at room temperature for 10 minutes.
Place sugar in 1 shallow dish and confectioners’ sugar in second shallow dish.
Use your hands to roll dough into 12 balls (about 2 tablespoons each). Drop balls directly into shallow dish with sugar and roll to coat. Transfer dough balls to shallow dish with confectioners’ sugar and roll to evenly coat.