2Combine flour, sugar, brown sugar, baking powder, and baking soda in a medium bowl. Mix well and set aside.
3In a large bowl blend together crushed pineapple (and juice), banana, egg whites, and vanilla using an electric mixer on low speed. Do not overbeat. Gradually add flour mixture and beat on medium high speed until all flour has been incorporated.
4In a small bowl, mix together chocolate chips, walnuts, and 1 tbsp flour. Fold into batter.
5Spray an 8x8" baking pan with non stick spray. Pour batter into pan. Bake for 40-45 minutes, until wooden pick inserted in middle comes out clean. Let cake cool completely in pan. Cut into 12 pieces and store at room temperature in an airtight container.