For Mary Pearl's second birthday, but her first with us, I went with a loose Valentine's theme, since her birthday is February 10 and she'd only been home since December. (Mama was brain-fried with another kid in the house! *ack*).
This is a square cake, and a circle cake cut in half to form the rounded portions of the heart:
The icing is still the store-bought icing, but for this it works great, because I wanted it to look really soft and swirly.