Preheat oven to 350F. Butter the bottom and sides of a 8X8 inch metal baking pan. Put a long piece of parchment paper in the bottom of the pan, letting the parchment extend up two sides of the pan and overhang slightly on both ends. Place another piece of parchment paper in the same manner, perpendicular to the first sheet. (This will make it easy to remove the bars from the pan after they have baked.)
In a medium bowl whisk together the flours and salt; set aside.
In a medium bowl, whisk together the melted butter and brown sugar until combined. Add egg, vanilla, espresso powder and mashed banana and mix well. Using rubber spatula, fold dry ingredients into the mixture until just combined; do not overmix. Fold in chocolate and nuts and turn batter into prepared pan, smoothing top with rubber spatula.
Bake until light golden brown, 22 to 25 minutes; do not overbake. Cool on wire rack to room temperature, about 2 hours, then remove blondies from pan by lifting parchment overhang. Cut into 2-inch squares and serve.