Preheat oven to 350 degrees. Melt chocolate in a heatproof bowl over a pan of simmering water until chocolate melts. Alternatively, you can melt the chocolate in a microwave on half power for a minute. Stir and return to the microwave every 30 seconds until melted. Set aside and let cool.
Stir together flour, cocoa powder, baking powder, and salt in a medium bowl. Set aside.
Mix softened butter and brown sugar with an electric mixer on medium speed until pale and fluffy, about 2 to 3 minutes. Mix in the eggs, one at a time, and mix only until combined. Add vanilla and melted chocolate. Mix until combined. Reduce the mixer speed to low and add the flour mixture in two batches, alternating with the milk. Divide dough into four equal pieces and wrap in plastic wrap. Refrigerate until firm, 2 hours or longer (can be refrigerated overnight).
Remove the dough, one at a time, working quickly, to form 12 to 16 one inch balls. The dough will melt and soften with the warmth of your hands, so it is important to work quickly, handling the dough as little as possible. I like to cut off small pieces with a butter knife and form into a ball, thereby handling the dough less. Place balls in granulated sugar to coat and then in the powdered sugar to coat. Place the balls on a parchment lined baking sheet 2 inches apart.
Bake until surfaces crack, about 12 to 14 minutes, depending on size. Let cool on sheets on wire racks. Store cookies in a airtight container between sheets of waxed or parchment paper.
Adapted from Martha Stewart Holiday Cookies
Recipe by Baking and Eggs at https://bakingandeggs.com/2017/02/12/chocolate-crackles/