Preheat your oven to 350°F (175°C) and grease a round cake pan.
In a mixing bowl, cream together the softened butter and sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition.
Stir in the mashed bananas and vanilla extract until combined.
In another bowl, whisk together the flour, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, alternating with the sour cream.
Mix until just combined; do not overmix.
If desired, fold in the chopped walnuts for added texture.
Pour the batter into the prepared cake pan and smooth the top.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.