PREPARE cake batter and bake as directed on package for 24 cupcakes, stirring chopped chocolate into batter before spooning into prepared muffin cups. Immediately poke 4 or 5 deep holes in each cupcake at 1/2-inch intervals, using round handle of wooden spoon.
MIX dry pudding mixes and sugar in medium saucepan; stir in milk and butter. Bring to full boil on medium heat, stirring constantly. Spoon over cupcakes.
REFRIGERATE 2 hours. Remove from pans. Frost cupcakes with COOL WHIP.