Two days before preparing the flan, toss the prunes with Armagnac or Cognac, cover securely and set aside to marinate. (If time is limited, you can skip the marinade and simply toss the prunes with the spirits, although the two-day marinade will offer a richer, more flavorful dessert.).
Preheat the oven to 375 degrees. Butter and flour a 10 1/2-inch straight-sided ceramic baking dish.
Add one tablespoon of sugar to the marinated prunes and toss. Place them on the bottom of the prepared baking dish, forming a single, tight layer that thoroughly covers the bottom of the dish.
Place the eggs and three tablespoons of sugar in a large bowl and, using a whisk or electric mixer, beat until well blended. Add the flour and mix well. Add the milk and mix well. Pour the batter into the prepared baking dish and bake until bubbly and brown, about 45 minutes.
Sprinkle with the remaining tablespoon of sugar and allow to cool. Serve at room temperature. This is just as delicious the next day. YIELD Eight to 10 servings. Wine recommendation:A young, sweet white wine from France's southwest, such as a Sauternes or a Barsac.