Prepare cake batter as directed on package except reduce the water to 3/4 cup. Add dry pudding mix and melted chocolate; mix well. Drop 2 Tbsp. batter, 2 inches apart, into 32 mounds on baking sheets.
Bake 14 to 16 min. or until toothpick inserted in centers comes out almost clean. Cool on baking sheets 3 min. Remove to wire racks; cool completely.
Beat cream cheese and marshmallow creme in large bowl with mixer until blended. Gently stir in COOL WHIP.
Place bottom sides of 2 cookies together for each whoopie pie, spreading about 2 Tbsp. COOL WHIP mixture between cookies.