Preheat the oven to 350 degrees F. Spray a 9-inch cake pan with nonstick cooking spray.

In a medium bowl, combine the dry ingredients: flour, kosher salt, and baking powder. Whisk to combine.

In the bowl of a stand mixer, beat the butter on medium speed until it is light and creamy. Slowly add the sugars to the bowl, and beat until well combined and creamy.

Scrape down the sides of the bowl with a rubber spatula. Add the eggs to the bowl, one at a time, beating between each addition. Add the pure vanilla extract and sour cream, beat to combine thoroughly.

With the mixer on low speed, slowly add the dry ingredients until just combined. Turn off the mixer, and use a rubber spatula to scrape the bottom of the bowl to ensure that all of the flour is incorporated. Add the extra 1 tablespoon of flour to the bowl of ¾ of the total peaches and blackberries. Use a spoon to gently coat the fruit with flour lightly. Fold the coated ¾ of the sliced peaches and blackberries into the batter, reserving some for the top of the cake.

Pour the batter into the prepared cake pan, and use a small offset spatula to smooth out the top. Arrange the remaining peaches and blackberries on the top of the cake. Bake for 60-70 minutes until the top of the cake is golden brown and a toothpick comes out with moist crumbs and feels warm to the touch. Allow the cake to cool for 30-40 minutes until slicing. Enjoy!