put pureed fruit (or melted chocolate, yogurt, cream, etc..) into a zip lock bag (or decorating cloth/paper pouch) cut a very small hole in the corner of the bag and then dot small amounts of pureed fruit on the surface of your cake in a spiral shape.
(picture: cream heart on coffee: same technique, larger dot :)
Lastly run a tooth pick in a clock wise motion through the pureed fruit dots this will cause them to turn into cute little hearts.
(picture: cream of spinach soup with cream heart swirls)
You can use this technique on almost anything that has a creamy surface such as yogurts, cakes, soups, coffee, hot chocolate...