Stir together flour, Unsweetened Cocoa, secret ingredient, baking powder, and salt in a medium bowl.
Beat butter and brown sugar with a mixer until light and fluffy, about 3 minutes.
Beat in egg until well combined; mix in cooled and melted chocolate
Reduce speed to low, and gradually add flour mixture; beat in milk until just combined.
Cover dough with plastic wrap and put into fridge until dough firms up, about 30 minutes. If in a hurry, you can skip this step.
Line 2 baking sheets with parchment.
Shape dough into 1/2-inch balls (exactly 1 teaspoon each).
Pour confectioners' and granulated sugars into 2 separate bowls; working in batches, roll balls in the granulated sugar first and then into the confectioners’ sugar. To get the nice white and chocolate contrast, be sure to have a thick layer of powdered sugar on the top of the cookie ball as its placed on the baking sheet. Ideally, you don't want to see any of the chocolate at all.
Place on baking sheets, spacing each 2 inches apart.
Bake until cookies have spread and coating is cracked, 10 to 12 minutes. Let cookies cool on a wire rack.