I wish I could say that I designed the logo specifically for the "penguin kiss", but this is one of those weird cases where things just sort of worked out by accident.
I did the logo first (in all black), and then searched the web for some suitable photo of penguins. I found this one at http://www.bay13.net.
My first trial run just had the penguin centered, so it appeared to be looking at the logo. Then I noticed that the design of the "y" of "Happy" looked a lot like a beak. I moved the penguin, and the "kiss" was born.
After printing it out and sending it, I made the "y" of "Birthday" look much more like a penguin head, and shifted the whole design down. But I still liked the original better, so I kept it.