Yesterday I made Easter cakes, inside egg shells...
I pierced the eggs and emptied the contents, and made the holes a bit bigger. After making the mixture, I used a piping bag to fill the eggs up about 3/4 full (although, after seeing how much spilled out during baking, I think 1/2 full would work).
To eat, crack the egg shell and marvel in the egg-shaped cakiness.
I got the idea here.
I followed the recipe there, but I didn't like the method at all. It seemed alien to mix the sugar and egg, and then add the butter to it - that's completely the wrong way round and didn't seem right. I don't think the cakes taste that brilliant, so if I did this idea again I would use a better cake recipe. And I'd mix the butter and sugar together first.
I think a little dollop of lemon curd, or butter cream, in the middle of the mixture would work really well.