Preheat oven to 350°F.
Melt butter in large cast iron skillet. Remove from heat and sprinkle brown sugar on top, evenly. Allow to cool slightly. Arrange sliced peaches and blueberries on top of butter and brown sugar. Set aside.
In large mixing bowl, cream together butter and granulated sugar until light and fluffy, about 5 minutes. Add eggs, one at a time, mixing well after each addition. Stir in vanilla extract.
In medium bowl, combine flour, baking powder and salt. Add dry ingredients to creamed butter and sugar in three batches, alternating with buttermilk. Make sure to start and end with dry ingredients, mixing just until incorporated after each addition.
Pour batter over the fruit, spreading evenly with rubber spatula. Bake until golden brown and set on top, about 40 to 45 minutes or until a toothpick inserted in middle of the cake comes out clean. Remove from oven and allow to cool down for about 10 minutes in pan. Run a knife along the edge and place a large plate or platter on top. Carefully invert cake. Serve warm or at room temperature. Enjoy!