I ended up making three round cakes which were iced and stacked on top of each other using dowels and boards. The entire cake was then given a final coat of yummy white chocolate buttercream to hide the separate layers and topped with a big blue fondant heart. The outside of the cake looked pretty and simple and I was told everyone loved the hidden hearts inside.
Here is a little update with a peak at the inside of the cake. (Thanks Steph for the photos)