HEAT oven to 350ºF.
PREPARE cake batter and bake in 24 paper-lined muffin cups as directed on package. Cool cupcakes in pans 10 min. Remove to wire racks; cool completely.
MEANWHILE, beat pudding mix and milk in medium bowl with whisk 2 min. Stir in 1/2 cup Cool Whip. Refrigerate until ready to use.
CUT cupcakes horizontally in half. Spoon 1 Tbsp. pudding mixture onto bottom half of each cupcake; cover with cupcake top.
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 chocolate is completely melted and mixture is well blended; spread onto cupcakes. Refrigerate 15 min.
serving size = 1 cupcake (55 g)