How to Make It
In a small bowl, soak the prunes in the Armagnac, stirring occasionally, until most of the liquid has been absorbed, 2 to 3 hours or overnight.
2. Preheat the oven to 400°. Lightly butter a shallow 6-cup baking dish. In a large bowl, whisk the flour with the sugar and salt, then make a well in the center. Crack the eggs into the well and add 1/2 cup of the milk. Using a whisk, beat the eggs with the milk, working in some of the flour. Gradually add the remaining 1 1/2 cups of milk and the melted butter while whisking in more of the dry ingredients; whisk until smooth.
Using a wooden spoon, work the batter through a fine sieve into the baking dish. Scatter the prunes in the dish and bake for about 35 minutes, or until risen and deep golden. Let cool slightly before cutting into rectangles. The flan can also be served at room temperature or rewarmed.