PREHEAT oven to 350ºF. Prepare cake batter and bake in 24 greased medium muffin pan cups as directed on package. Cool 10 min. in pans. Remove to wire racks; cool completely.
POUR milk into large bowl. Add dry pudding mix. Beat with wire whisk 2 min. or until well blended. Let stand 5 min. to thicken. Meanwhile, using a serrated knife, cut cupcakes horizontally in half. Gently stir 1/2 cup of the whipped topping into pudding. Spoon about 1 Tbsp. of the pudding mixture onto bottom half of each cupcake; cover with top of cupcake.
MICROWAVE remaining 1 cup whipped topping and the 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. Frost each cupcake with about 1-1/2 tsp. of the chocolate mixture. Refrigerate at least 15 min. before serving. Store leftover cupcakes in refrigerator.