Heat oven to 350°F. Line 12 muffin cups with paper baking cups. In medium bowl, combine cake mix and water; stir until smooth. Spoon batter evenly into paper-lined muffin cups.
Bake at 350°F. for 17 to 22 minutes or until cupcakes springs back when touched lightly in center.
Meanwhile, with scissors dipped in water, cut each marshmallow heart candy in half horizontally.
Immediately place 1 marshmallow half, cut side down, on top of each hot cupcake.