How to Make It
Preheat the oven to 350°. In a 12-inch ovenproof skillet, melt 4 tablespoons of the butter. Sprinkle the brown sugar evenly in the skillet. Turn off the heat and arrange the plum wedges in the skillet in 2 concentric circles.
In a small bowl, whisk the flour, baking powder and salt. In a medium bowl, using a handheld mixer, beat the remaining stick of butter with the granulated sugar until fluffy. Beat in the egg yolks, 1 at a time, until incorporated, then beat in the vanilla. Beat in the dry ingredients in 3 additions, alternating with the milk.
In a medium bowl, beat the egg whites until they hold firm peaks. Using a rubber spatula, stir one-third of the egg whites into the batter to lighten it, then gently fold in the remaining whites. Spread the batter over the plums and bake for about 55 minutes, or until a cake tester inserted in the center comes out clean. Let the cake cool for 15 minutes, then turn it out onto a plate. Cut the cake into wedges and serve warm or at room temperature.