Chop the chocolate up into small pieces and add to a small microwaveable bowl. Use the chocolate setting of your microwave to melt it in 20 second increments, stirring in between, until completely melted. Set aside to cool slightly.
In a separate bowl whisk together the flour, cocoa, baking powder and salt.
In a separate large bowl beat the butter and brown sugar together on medium speed until light and fluffy about 3 minutes. Beat in the eggs and vanilla until well combined. Beat in the melted chocolate.
On low speed beat in half the flour mixture followed by all the milk. Scrap down the sides of the bowl and beat in the remaining flour. Cover with plastic wrap and refrigerate 2 hours or overnight.
Once the dough is well chilled preheat oven to 350 degrees F. Line 2 large baking sheets with silpats or parchment paper.
Add the confectioners' sugar to a shallow plate with sides. Scoop up rounded tablespoons of dough with a cookie scoop and drop into the sugar, coat them well and form into a ball. If there's any of the chocolate showing through roll them again in the sugar.
Place 2 inches apart on the cookie sheet and bake 13 - 14 minutes until the balls flatten and the sugar splits to form the "crackly pattern". If the dough starts to soften too much in between batches then refrigerate it.
Cool cookies on baking sheets 5 minutes then transfer to wire racks to cool completely. Store in a tightly covered container up to a week.