Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.
In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set aside.
In a large bowl, cream the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Mix in the mashed ripe bananas and sour cream until combined.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
Fold in the chopped walnuts if using.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and let it cool in the pan for 10 minutes. Then transfer to a wire rack to cool completely.
To make the cream cheese frosting, beat the softened cream cheese and butter together until smooth and creamy.
Add the powdered sugar gradually, beating until smooth and fluffy.
Stir in the vanilla extract.
Once the cake is completely cooled, spread the cream cheese frosting evenly over the top.
Slice and serve.