Melt chocolate in a heatproof bowl set over simmering water or in the microwave at 60% power in 30-second intervals. Stop when chocolate is 80% melted and stir to finish melting it. Set aside and cool to room temperature.
In a medium bowl, whisk together flour, cocoa, baking powder and salt.
With an electric mixer on medium-high speeed, beat butter and brown sugar until light and fluffy. Beat in egg, then vanilla and chocolate. Reduce mixer speed to low and beat in flour mixture. Chill dough until firm.
Preheat oven to 375ºF. Scoop tablespoonfuls of dough and roll between your palms to form smooth balls.
Place confectioners’ sugar in a shallow dish. Roll dough balls generously in the confectioners’ sugar (you want a thick coating).
Place dough balls on parchment-lined baking sheets about 2 inches apart. Bake about 10 minutes, until tops are cracked and centre is almost set. Cool 3 minutes on baking sheet before transferring to wire racks to cool completely.
Notes
To make ahead: dough may be stored up to 3 days in the refrigerator before baking.