Melt chocolate in a heatproof bowl set over simmering water. Set aside and cool to room temperature.
In a medium bowl, whisk together flour, cocoa, baking powder and salt. Set aside.
With an electric mixer on medium-high speeed, beat butter and brown sugar until light and fluffy. Beat in eggs, then chocolate. Reduce mixer speed to low and beat in flour mixture. Transfer bowl to freezer to chill for 30-60 minutes, until dough is firm enough to handle. To make ahead: dough may be stored up to 3 days in the refrigerator before baking.
Preheat oven to 350 degrees F. Place confectioners’ sugar in a shallow dish. Roll tablespoonfuls of dough between your palms to form balls, then roll very generously in the confectioners’ sugar (you want a thick coating). Place cookies on greased or 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.