I've made these a few times with just box cake and they turned out spectacularly! I recommend putting a little bit of oil on the outside of the egg to keep the cake from overflowing and sticking to the shell. You can keep them in the shell or peel it, depending on the affect you wish for!
If you use white cake, you can mix in food coloring to make them more festive!
A detailed and graphical description of this process can be found here:
((if you have other unique ideas such as this, please share them with me, I absolutely LOVE this stuff--like gummy vodka bears ))