Sift the flour into a large baking bowl, make a dent in it and pour in the yolks and salt. Stir everything together and add as much milk as necessary to make a thick batter. Finally, add the Armagnac. Whip the egg whites with the sugar until very stiff and fold into the batter just before baking. Preheat the oven to 180 °C.
Remove the seeds from the plums, cut them in half, mix them loosely into the batter and pour the amount into a well-buttered tart springform pan. Spread the butter flakes evenly on the surface and bake the casserole in the heated oven at 180 °C for 40-45 min. Best served still lukewarm with vanilla sauce.