Preheat your oven to 350 degrees.
Grease and flour a 9-inch round cake pan.
Start by adding the bananas to the bowl of an electric mixer fitted with the paddle attachment. Mash the bananas with a fork.
Next, add in the granulated sugar, and brown sugar and mix on low speed until combined.
With the mixer still on low, add the oil, eggs, sour cream, and vanilla. Mix until smooth.
In a separate bowl, sift together the flour, baking soda, and salt.
With the mixer on low, add the dry ingredients to the banana mixture and mix just until combined.
Pour the batter into the prepared pan and bake for 45 to 50 minutes, until a toothpick inserted in the center comes out clean.
Cool in the pan for 15 minutes. Then turn out onto a cooling rack, and cool completely.
Spread the frosting thickly on the top of the cake and decorate with walnut halves.