Melt 1 tablespoon butter in a 10-inch cast-iron skillet over medium heat; sprinkle brown sugar evenly in pan. Remove from heat. Arrange peach slices in a circle in center of pan; scatter blueberries and raspberries evenly around peach slices. Set aside.
Place remaining 1/4 cup butter and granulated sugar in a large bowl; beat with a mixer at medium speed until well blended. Add rind, juice, egg whites, and egg; beat well. Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, baking soda, and salt, stirring well with a whisk. Add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture.
Carefully spoon batter over fruit mixture. Bake at 350° for 35 minutes or until a wooden pick inserted in the center comes out clean. Run a knife around the edges of pan. Cool cake completely in pan on a wire rack. Place a plate upside down over skillet; carefully invert onto plate.
Very good cake and pretty to look at, too. I used peaches and strawberries, and cut the sugar in the cake to 1/2 cup. I used powdered sugar on the cut pieces, which made for a nice presentation, though a dollop of whipped cream or vanilla ice cream might add a lot. I also added a tablespoon of wheat germ to the flour for a little more character.
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