TRIM rounded edges from cake; discard trimmings or reserve for snacking. Cut cake into 24 (1/4-inch-thick) slices.
BEAT pudding mix and milk with whisk; spread onto half the cake slices. Cover with remaining cake slices. Cut in half. Place in single layer on waxed paper-covered baking sheet.
MICROWAVE frosting in microwaveable bowl on HIGH 35 sec.; stir. Pour over cake pieces. Refrigerate 30 min. or until firm.