Here's the recipe:
1. Foam an egg white till firm
2. Butter a ceramic or aluminium small pan
(You can also find it online
3. fill it to the edge with the beaten egg white, molding a little space to put the yolk in
4. put the yolk in and cover it with the remaining egg white, shaping it like a balloon
5. bake it in a pre-heated oven at 200°C for 6 to 10 minutes - depending on the oven - until it is brownish and hard.