Preheat oven to 350 degrees.
Spray a 9-inch round cake pan with cooking spray and set aside.
In the bowl of a stand mixer, whip together the butter and 1 cup of the sugar until pale and fluffy, about five minutes.
Add the eggs, one at a time, and beat well after each addition. Beat in the vanilla.
In a medium bowl, whisk together the flour, baking powder and salt. Gradually add the flour mixture to butter mixture alternately with sour cream, beginning and ending with flour mixture, beating just until combined after each addition.
Gently fold in the fresh blueberries and raspberries to the cake batter using a rubber spatula. Spread the batter in the prepared cake pan.
Sprinkle the remaining 1/4 cup of sugar evenly over the top of the cake batter.
Bake 35-40 minutes, until the top is crackly and lightly browned and the center still jiggles just a bit.
Let the cake cool completely in the pan and slice and serve the cake from the pan.
Top each cake slice with whipped cream and more fresh blueberries and raspberries.