Preheat your oven to 375 degrees Fahrenheit and grease a 9-inch by 13-inch glass baking dis, lining it with parchment paper.
In a large bowl, use a hand mixer (or in the bowl of your stand mixer) cream the butter and sugar for at least 4-5 minutes.
Add the egg, vanilla and sour cream and beat very well until well blended.
Add in the flour, baking powder, and salt and mix with the hand mixer on low speed until the mixture reaches a wet, crumbly texture.
Press this mixture into the prepared baking dish and bake at 375 degrees Fahrenheit for about 17-20 minutes or until just before the edges start to brown.
Add the powdered sugar and vanilla and continue whipping on high speed until the frosting becomes soft and spreadable.
Drizzle in the whipping cream while whipping, only if necessary, until the frosting is soft and spreadable.
Remove from the pan using the parchment paper, arrange the berries on top, slice and serve immediately.
To make these bars ahead of time, simply add the cheesecake topping and store in an airtight container in the refrigerator, topping with berries right before serving.