I saw these egg shell cakes and wanted to make them for Easter. Here was my try. The cake was just box cake. It was pretty easy, but very time consuming to get the eggs out of the shells. My holes were some thing like a 1/4" wide. I decided to save the egg to use for breakfast so I had to keep eggshell out of them. It helped to kind of shake the egg in order to get it out, and much easier once the yoke broke.
(My young daughters had decorated the shells before I took the eggs out, which was crazy itself because I was very nervous about the girls breaking the shells.)
Then I mixed the batter and piped it into the greased shell. The greasing was just spraying cooking oil into it, and knowing it would be enough coverage. I did open the hole in the shell up a bit more for piping. It was very difficult to tell when 2/3 of the shell was full. I am think more like 1/2 should be full in this case. It was easy to take the batter off the outside of the shell, but it ruined any of the decoration on the outside.
Then I decided to just break off a bit more of the shell to help people to eat them and top with whipped cream. It was a lot of work to make, and a lot of work in order to eat. I would not make them again unless there was a very special occasion which called for these very cute cakes.