For the Plum Mixture:

Prepare the pans: Grease the bottom and sides of a 9-inch square or round, 3-inch deep metal cake pan. If using a 10″ cast iron skillet, no need to grease.

Melt butter in a medium saucepan over medium heat; add brown sugar and cook, stirring occasionally, until the mixture is foamy and pale, 3 to 4 minutes. Pour mixture into prepared pan; distribute evenly. If using cast iron, simply melt the butter in, brushing a little over the sides. Add the brown sugar and cook as above.

Arrange the fruit over topping; set aside. Working in concentric circles is not necessary, but it looks nice.

For the Cake:

Position the oven rack in the lower middle position and heat oven to 350 degrees F.

Whisk flour, cornmeal, baking powder, and salt in a medium bowl. In another bowl, cream the butter. Gradually add the cup of sugar. Beat until light and fluffy, about 2 minutes.

Beat in yolks and vanilla. Reduce speed to low and add 1/3 of the dry ingredients, mixing well, then half the wet, mixing well. Follow with another 1/3 of the dry and the remainder of the wet, mixing after each. Finish with the final 1/3 cup dry and mix until batter is smooth. Set aside.

Using an electric mixer, beat the egg whites in a large bowl at low speed until frothy. Increase speed to medium-high; beat to soft peaks. Gradually add remaining two tablespoons of sugar; continue to beat into stiff peaks. Gently fold a quarter of the egg whites into the flour mixture to lighten, then fold in remaining egg whites until just combined.

Pour batter into pan and spread evenly on top of the fruit. Bake until top of the cake is golden and a toothpick comes out clean, 60 to 65 minutes.

Rest cake on wire rack for two minutes for a cake pan, about five for a cast iron skillet. Slide a pairing knife around the edge of the cake to loosen it from the pan. Place a serving platter over the pan and holding tightly, invert the cake onto the platter. Carefully remove the cake pan. If any fruit sticks to pan or falls off cake, carefully reposition.