Combine prunes and Armagnac in a small bowl and macerate (soak) for 2 hours, stirring occasionally. Drain, reserving the liquid. Preheat oven to 350 degrees. Lightly oil an 8-inch round cake pan or spray with nonstick cooking spray. Line the bottom of the pan with a circle of parchment or wax paper. Lightly oil or spray the paper and dust the pan with cornmeal, shaking out the excess. In a medium-sized bowl, combine cornmeal, flour, baking powder, and salt. In a large mixing bowl, whisk together sugar, oil, butter until well combined. Add egg and egg white, whisking until just combined. Stir in yogurt, vanilla and 1 tablespoon of the reserved prune soaking liquid. (Set aside the remainder for brushing the top of the cake.) Fold in the dry ingredients until just combined. Pour the batter into the prepared pan and scatter the reserved prunes over the top. Bake for 20 to 25 minutes, or until the cake is golden and a toothpick inserted in the center comes out clean. Cool on a rack 10 minutes. Invert the cake, peel off the paper, and place right-side up on a serving plate. (The cake can be cooled, wrapped in plastic and refrigerated for up to 2 days. Warm for 10 minutes at 350 degrees before serving.) Brush the top of the cake with the remaining prune soaking liquid and serve warm. Per Serving: 232 calories; 3g protein; 6g fat; 35g carbohydrates; 79mg sodium; 31mg cholesterol.