Preheat oven to 350 degrees. In a large bowl, melt chocolate and butter in the microwave on high in 30-second increments, stirring after each, until chocolate is almost melted. Stir to finish melting; let cool.
In a separate bowl, whisk together flour, cocoa, baking soda, and salt. Add brown sugar, egg, molasses, and vanilla to the cooled chocolate
mixture, whisking to combine. Add flour mixture, and stir to combine.
Scoop up dough by the tablespoon, and roll into balls; place 2 inches apart on an ungreased baking sheet. Bake until tops are crackly but cookies are still soft to the touch, about 12 minutes. Let cool 5 minutes, then transfer to a wire rack to cool
completely. Dust with confectioners' sugar, if desired.