HEAT oven to 350°F.
MICROWAVE chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted. Stir in sugar. Add eggs and vanilla; mix well. Add milk, flour and dry pudding mix; stir until well blended. Spray 2 muffin pans with cooking spray. Spoon 2 Tbsp. chocolate mixture into each muffin cup, filling about half full.
BAKE 16 to 18 min. or until toothpick inserted in center comes out with fudgy crumbs. Top each cupcake with 1 marshmallow; press down with back of teaspoon. Bake an additional 2 to 3 min. or until marshmallows are soft. Place muffin pans on wire rack. Press marshmallow down again with back of teaspoon.
COOL 10 min. Carefully run table knife around edge of each cupcake; remove to wire rack. Cool completely.