Beat the egg with a little lemonade until frothy. Add the flour and, if necessary, a dash of lemonade and beat vigorously. Let the dough rest for about 30 minutes.
Halve and core the plums and cut into thin wedges.
Heat a trick of butter in a pan, pour in half of the dough, top with the prunes and pour the other half of the dough over it. Bake the pancake until golden on both sides and serve.