For the galettes, mix buckwheat flour, eggs, salt and milk or water to a dough. Pour in the melted butter, stirring constantly. Cover the dough and let it rest in a cool place for at least 1 hour.
Heat a little butter in a non-stick skillet. Pour in a small ladleful of batter and swirl the pan to allow the batter to rise nicely. Since the galettes are baked very thin, there is no need to turn them.
Lift out the galette and place on a plate. Bake the remaining dough to make galettes. Fill the galettes as desired, fold twice and serve immediately.