Beat the eggs with sugar, add the flour and baking powder and then vanilla. Put a baking sheet in a square baking dish greased with oil, pour the mixture in and bake at 392°F (200 °C).
Once baked, roll up the still warm layer with a well moistened cloth and leave it for a few minutes to acquire its shape.
Unroll gently and smear it with jam. The decoration is optional :)