Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan.
In a large bowl, cream together the softened butter and sugar until light and fluffy.
Add in the mashed bananas, mixing well to combine.
Beat in the eggs, one at a time, followed by the vanilla extract.
In a separate bowl, whisk together the flour, baking soda, and salt.
Gradually add the dry ingredients to the banana mixture, alternating with the sour cream.
Mix until just combined; do not overmix.
If desired, fold in nuts or chocolate chips.
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.