You are not wrong. There is a major cake deficit in the Loop -- although Bake for Me does a cake that will serve but is not particularly fabulous.
You can purchase a cake at Macy's in the basement -- it depends on the day what's available.
You can high tail it over to Fox n Obel -- they have cakes of several different varieties but they're not exactly in the Loop.
My advice to you, as a woman who handles the birthdays in her office, is to order from your favorite bakery and have it delivered. I've done that from Alliance and Dinkels and i don't think the delivery charges were really out of line. And there is nothing better than a really really good cake when it's about 3:30 and you've had one of those, "oh my god, where is the CAKE?" days.
In fact..what time is it now????
Shannon