Preheat oven to 375°F (190°C). Grease and flour a 9x13-inch baking pan.
In a large mixing bowl, beat together sugar, sour cream, softened butter, and eggs until smooth and creamy.
Add mashed bananas and vanilla extract. Mix until fully combined.
Stir in flour, baking soda, and salt. Mix just until combined.
Fold in chopped walnuts if using.
Spread the Banana Bread Brownies batter evenly into the prepared pan.
Bake for 20–25 minutes or until the top is golden brown and a toothpick inserted in the center comes out clean.