HEAT oven to 350°F.
LINE 8-inch square pan with foil, with ends of foil extending over sides; spray with cooking spray.
WHISK first 4 ingredients in large bowl until well blended. Mix flour, baking powder and baking soda. Add to butter mixture; stir until well blended. Add chocolate; mix well. Spread onto bottom of prepared pan.
BAKE 25 min. or until toothpick inserted in centre comes out clean. Cool completely.
USE foil handles to lift dessert from pan; remove foil. Microwave caramels and milk in small microwaveable bowl on MEDIUM 2 min. or until caramels are completely melted and mixture is well blended, stirring after each minute. Drizzle over dessert. Cool 20 min. or until caramel is firm. Cut into squares.