1. Preheat the oven to 350°F (180°C). Lightly grease a 12-cup muffin tin. In a large bowl, combine flour, cocoa powder, baking powder and salt. Using an electric mixer, beat together butter, granulated sugar and brown sugar until light and fluffy; beat in egg. Stir in flour mixture until combined. Refrigerate for 1 hour.
2. Roll dough into 2-inch (5 cm) balls and place in prepared muffin cups, pressing into the bottom and upside to create an indent. Bake for 13 to 15 minutes or until centres are just set. Let cool completely in pan on rack.
3. In a microwaveable bowl, microwave chocolate chips on Medium for 2 minutes; stir and return to microwave, stirring every 30 seconds, until melted and smooth. Using a pastry brush, coat the interior of cookie cups with chocolate. Refrigerate for about 10 minutes or until set.
4. Remove cookie cups from muffin tin; fill with cold milk.