- 12 tablespoons unsalted butter, softened, divided
- 3/4 cup brown sugar
- 1 teaspoon aged balsamic vinegar
- 1/2 teaspoon plus a pinch of salt, divided
- 2 cups frozen pie cherries
- 1 1/3 cups all-purpose flour
- 1/4 cup whole wheat pastry flour
- 1 1/2 teaspoons baking powder
- 1 cup plus 2 tablespoons granulated sugar, divided
- 4 large eggs, separated
- 1 teaspoon vanilla extract
- 2/3 cup buttermilk
Preheat oven to 350° F.
Melt 4 tablespoons butter in a 10-inch cast iron skillet over medium heat. Add brown sugar; cook, stirring occasionally until foamy, about 2 minutes. Stir in vinegar and a pinch of salt; cook for 30 seconds. Remove from the heat, add cherries and arrange in an even layer across caramel.
Combine flours, baking powder and ½ teaspoon salt in a bowl. Beat remaining 8 tablespoons butter in a stand mixer until fluffy, about 1 minute. Beat in 1 cup granulated sugar and mix for 3 minutes on medium speed. Beat in yolks and vanilla. At low speed, add half the dry ingredients, then half the buttermilk; repeat with remaining half of each. Mix just until smooth.
In a clean bowl, beat egg whites until frothy. Slowly add remaining 2 tablespoons sugar and beat until stiff peaks form. Using a spatula, fold about 1/4 of egg whites into batter. Fold in remaining whites until there are no streaks. Pour batter into the pan over fruit.
Bake until the top is golden, about 1 hour. Let cake cool for 5 minutes. Loosen from the sides of the pan with a paring knife, then carefully invert onto a cake plate. Serve warm or at room temperature.