HEAT oven to 350ºF.
PREPARE cake batter and bake in 24 paper-lined muffin cups as directed on package. Cool in pans 10 min. Remove to wire racks; cool completely.
BEAT pudding mix and milk in large bowl with whisk 2 min. Let stand 5 min. Meanwhile, cut cupcakes horizontally in half. Gently stir 1/2 cup Cool Whip into pudding; spoon 1 Tbsp. onto bottom half of each cupcake. 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 chocolate is completely melted and mixture is well blended; spread onto cupcakes. Refrigerate 15 min.
serving size = 1 cupcake (55 g)