1/2 cup sugar – 1/4 teaspoon pure vanilla extract
1/4 teaspoon salt – 5 tablespoons unsalted butter, melted and cooled, plus more for pan
3/4 cups all-purpose flour – 1 cup good-quality pitted prunes (for instance, pruneaux d’Agen)
1/4 cup Armagnac plus 1/4 cup water or 1 cup hot Earl Grey tea
Confectioners’ sugar, for dusting.
In a blender, combine the milk, eggs, sugar, butter, vanilla, and salt and blend for 1 minute. Add the flour and pulse until just incorporated, scraping down the sides of the blender jar. Chill in the jar for at least three hours and up to one day.For Armagnac-soaked prunes, combine the fruit and water in a small saucepan. Simmer over medium heat until the fruit is softened and water is almost evaporated, stirring occasionally. Turn off the heat and pour the brandy evenly over the fruit. Stand back and ignite the alcohol with a long match. Once the flame dies out, transfer the fruit and syrup to a bowl. Allow to come to room temperature, cover and set aside. For tea-soaked prunes, place the fruit in a heatproof bowl and add the hot tea. Allow to come to room temperature, cover and set aside. Steep prunes overnight.
Position rack in the center of oven and preheat to 375 degrees. Butter an 8-inch round cake pan. Line bottom of the pan with parchment or waxed paper, butter the paper then dust the pan with flour, tapping out excess.
Blend the batter again until smooth, about 5 seconds. Pour into the prepared cake pan. Drop the prunes evenly into batter. Place cake pan on a baking sheet and bake for 50 to 60 minutes, until sides are browned and puffy and knife inserted into center comes out clean.
Cool cake completely on a cooling rack. Loosen cake from the pan by running a knife around the sides. Carefully invert pan onto a piece of wax or parchment paper, remove the pan and peel off parchment round. Place serving plate over cake and invert again. Dust cake with powdered sugar and serve.