HEAT oven to 350ºF. Prepare cake batter and bake in 24 greased muffin pan cups as directed on package. Cool 10 min. in pans. Remove to wire racks; cool completely.

BEAT milk and pudding mix in large bowl with whisk 2 min. Let stand 5 min. Meanwhile, cut cupcakes horizontally in half. Stir 1/2 cup Cool Whip into pudding; spoon 1 Tbsp. onto bottom half of each cupcake. Cover with tops of cupcakes.

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. Refrigerate leftovers.