Whisk together the flour, baking powder, and salt and set aside.
Cream the soft butter and sugar together in a stand mixer for 3-4 minutes, until light and fluffy. Scrape down the side of the bowl a couple of times.
Beat in the eggs, one at a time, and then beat in the vanilla.
Stir the sour cream and buttermilk together to blend, and then add the flour to the mixing bowl alternately with the sourcream/milk, beginning and ending with the flour. Mix until combined, but don't over mix.
Fold in the berries and turn into the prepared pan. Smooth out the top. Sprinkle the surface of the cake liberally with granulated sugar.
Bake for 40-45 minutes, or until the cake no longer jiggles in the center and the top is golden and slightly crackled. The exact cooking time will depend on the pan size you use. You can insert a toothpick in the center to test.
Let cool briefly, and then unlatch the spring and remove the outer ring. I like to run a spreading knife along the edge first to loosen any parts of the cake that are sticking to the pan. Cool completely on a rack before slicing.
I used half sour cream and half buttermilk, but you can use all sour cream, all buttermilk, half and half, full fat yogurt, or full fat milk for this cake.
You can bake this in a 9 inch cast iron skillet. Serve it right in the pan.
Try it with other fruit, especially other berries like blueberries, raspberries, or blackberries.