CHOP 6 oz. chocolate; place in medium microwaveable bowl. Add butter. Microwave on MEDIUM 2 min. or until butter is melted; stir until chocolate is completely melted. Stir in sugar. Add eggs, 1 at a time, mixing well after each. Add combined flour and baking powder; mix well. (Dough will be very soft.) Refrigerate 2 hours.
HEAT oven to 350°F. Roll rounded tablespoonfuls of dough into 34 balls, each about 1-1/4 inches in diameter. Place, 2 inches apart, on baking sheets sprayed with cooking spray.
BAKE 8 to 9 min. or until no longer wet and shiny. (Cookies will still be soft.) Immediately top each cookie with marshmallow half, cut-side down. Bake 2 min. or until marshmallows start to puff. Cool 5 min. on baking sheets; transfer to wire racks. Cool completely.
CHOP remaining chocolate; place in small microwaveable bowl. Microwave on MEDIUM 2 min.; stir until completely melted. Stir in milk until well blended. Drizzle over cookies. Refrigerate 15 min. or until chocolate is firm.