FOR THE CAKE:
Preheat the oven to 350° F.
Spray a 9-inch pie plate with cooking spray.
In a small bowl , mix the flour, baking powder, baking soda, and salt. Set it aside.
In a large bowl, use a wire whisk to beat the sugar, melted butter, and sour cream.
Whisk in the egg and vanilla extract.
With a spoon, stir in the flour mixture until smooth.
Gently stir in 1 cup of the mixed berries. The batter is quite thick.
With a spatula, spread the batter evenly in the pie plate, smoothing the top.
Sprinkle the remaining ½ cup of berries over the top. Sprinkle the top with 1 teaspoon of sugar.
Bake at 350° F. for 35 to 40 minutes, or until a cake tester comes out clean.
Place the pie pan on a wire rack to cool for 20 minutes.
FOR THE WHIPPED SOUR CREAM:
With an electric mix at high speed, beat all the TOPPING INGREDIENTS for 1 minute, just until soft peaks form. Do not overmix! With a spatula, spread it on top of the cake.
Store the cake covered in the refrigerator.