PREPARE cake batter and bake as directed on package for 24 cupcakes. Cool completely.
BEAT pudding mix and milk with whisk 2 min. Let stand 5 min. Meanwhile, use serrated knife to cut cupcakes horizontally in half. Whisk 1/2 cup COOL WHIP into pudding; spoon onto bottom halves of cupcakes, using about 1 Tbsp. for each. Cover with cupcake tops.
MICROWAVE remaining COOL WHIP and chocolate in small microwaveable bowl on HIGH 1-1/2 min. or until chocolate is almost melted, stirring after 1 min. Stir until well blended. Let stand 15 min.; spread onto cupcakes. Refrigerate 15 min.