Step 1: Preheat your oven to 350°F (175°C). This ensures that your bars bake evenly.
Step 2: Line a 9x13 inch baking pan with parchment paper, leaving some overhang for easy removal later.
Step 3: In a large mixing bowl, cream together the softened butter and sugar until light and fluffy. Add the sour cream, eggs, mashed bananas, and vanilla extract. Mix until fully combined.
Step 4: In another bowl, whisk together the all-purpose flour, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined. If desired, fold in the chopped walnuts.
Step 5: Pour the batter into the prepared baking pan and spread it evenly. Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.
Step 6: Allow the bars to cool in the pan for about 10 minutes before transferring them to a cooling rack to cool completely.
Step 7: In a mixing bowl, beat together the softened butter, powdered sugar, vanilla extract, and milk until smooth and creamy. Adjust the milk to achieve your desired consistency.
Step 8: Once the bars are completely cool, spread the cream cheese frosting over the top. Cut into squares and enjoy!