Preheat your oven to 350°F. Grease a 9x13 inch baking dish.
In a large bowl, beat 1/2 cup softened butter and sugar until creamy. Add eggs, sour cream, and 1 tsp vanilla. Mix until smooth.
Stir in mashed bananas. In another bowl, whisk together flour, baking soda, and salt.
Gradually add dry ingredients into the banana mixture. Stir until just combined. Fold in walnuts if using.
Pour batter into the prepared pan. Bake for 25–30 minutes or until a toothpick comes out clean. Cool completely.
For the frosting, beat cream cheese, 1/2 cup softened butter, and 1 tsp vanilla until smooth. Gradually add powdered sugar and beat until fluffy.
Spread frosting generously over the cooled cake. Slice and serve (preferably with coffee).