DirectionsPreheat the oven to 350 degrees.
Melt 4 tablespoons of butter with the brown sugar, and pour into a 10-inch round cake pan.
Coat the bottom of the pan evenly with the mixture.
Quarter the plums and arrange them neatly to cover the bottom of the pan.
Cream the remaining butter with the sugar until pale and fluffy.
Add the vanilla extract, and beat in the egg yolks one at a time.
Sift together the flour, baking powder and salt, and add them to the batter alternately with the milk, beginning and ending with the dry ingredients.
Whisk the egg whites until they hold a slight shape.
Fold the egg whites into the batter a third at a time until incorporated.
Pour the batter into the prepared pan, and spread it evenly over the plums.
Bake for 45 to 55 minutes, until the top is browned and a cake tester inserted in the center comes out clean.
Let the cake cool for 20 minutes, and then turn over onto a cake plate.
Serve with whipped cream.