ADD boiling water to gelatin mix in medium microwaveable bowl; stir 2 min. until completely dissolved. Stir in marshmallows. Microwave on HIGH 1 min. or until marshmallows are puffed; whisk until marshmallows are completely melted and mixture is well blended.
POUR into 9-inch square pan sprayed with cooking spray.
REFRIGERATE 45 min. or until firm. (Mixture will separate as it sits.)
RUN knife around edge of pan to loosen gelatin. Cut gelatin into 32 heart shapes using 1-inch cookie cutter. Garnish with sprinkles.