Preheat the oven to 350 degrees F. Line a 9x13-inch baking pan with foil. leaving a 2-inch overhang on two sides. Coat the foil with nonstick spray.
In a medium bowl, whisk together the flour and salt.
In a large bowl, use an electric mixer to combine the butter and sugars until creamy- about 2 minutes. Mix in the eggs, banana and vanilla; beat until combined. Beat in the flour mixture until just combined, then stir in the chocolate chips and walnuts with a wooden spoon.
Spread the batter in the pan with a rubber spatula. Bake until a toothpick inserted into the center of the bars comes out clean- 35 to 40 minutes. Remove the pan from the oven and let the bars cool completely.
Lift the bars out of the pan using the overhanging foil. Peel off the foil. Cut into squares.