Preheat oven to 180 C
Melt chocolate and then set aside to cool.
In a medium bowl, sift together flour, cocoa, baking powder, and salt
In the bowl of an electric mixer fitted with the paddle attachment, beat butter and brown sugar on medium speed until light and fluffy, 3 to 4 minutes
Add eggs and vanilla, and beat until well combined
Add melted chocolate.
Add flour mixture alternately with milk.
Mix on low speed until just combined
Shape dough into a flattened disk and wrap with plastic wrap
Chill in the refrigerator until firm, about 2 hours
Preheat oven to 350
Line two baking sheets with parchment paper; set aside
Using 1 heaping teaspoon of dough apiece, shape 1-inch balls
Roll each in white sugar and then confectioners’ sugar until completely coated. If any cocoa-colored dough is visible, roll dough in confectioners’ sugar again.
Place the cookies on prepared baking sheets, 2 inches apart. Bake until flat and the sugar coating splits, 12 to 15 minutes, rotating halfway through
Transfer to a wire rack, and let cook completely