In a double-boiler over medium-high heat, melt butter and chocolate until smooth. Remove from heat and cool slightly. In a large bowl with an electric mixer, beat sugar and eggs until smooth. Beat in vanilla and melted chocolate mixture.

In a large bowl, whisk flour, baking powder, baking soda, salt, and cocoa. Beat dry mixture into liquid until mixture forms a dough. (dough will be very soft and sticky) Wrap dough in plastic wrap and refrigerate 1 hour.

Remove chilled dough from fridge and roll into 1-in balls. Roll balls in sugar and place on parchment paper-lined baking sheets. Flatten dough ball slightly. Bake cookies at 350F 9-10 minutes. (cookies will appear very soft and underdone--this is ok) Cool cookies on baking sheet 10 minutes then transfer to a wire cooling rack to cool completely.