Preheat oven to 350°F. Grease a 10-inch cake pan with nonstick cooking spray and line with parchment.
Melt butter in a large skillet over medium heat. Add brown sugar and salt and cook, whisking frequently until sugar caramelizes and darkens slightly, about 3-4 minutes. Add Banana Berry and cook, stirring frequently until coated, about 1-2 minutes.
Pour fruit mixture in an even layer into the prepared cake pan. Set aside.
For the cake:
In a large bowl, whisk butter, sugar and vanilla together until combined. Whisk in one egg at a time until light and fluffy. Add sour cream, baking powder, salt and baking soda and beat until combined. Fold in flour until just combined (batter will have lumps).
5. Pour batter into prepared cake pan. Bake until a toothpick inserted into the center comes out clean, about 35-40 minutes.
6. Gently run a small knife around the edge of the cake to help loosen from cake pan. Invert cake pan onto cooling rack to release cake. Top with walnuts. Serve at room temperature with ice cream.