Preheat oven to 375 degrees. Lightly grease cookie sheets. Peel banana and place in small bowl. Mash enough banana with fork to measure 1/2 cup. Set aside. Place flour, baking powder and salt in small bowl; stir to combine. Beat butter, granulated sugar and brown sugar in large bowl with electric mixer at medium speed until light and fluffy, scraping down side of bowl once. Beat in banana, egg and vanilla, scraping down side of bowl once. Add flour mixture. Beat at low speed until well blended, scraping down side of bowl once. Stir in chips and walnuts with mixing spoon. (Dough will be soft) Drop rounded teaspoonfuls of dough 2 inches apart onto prepared cookie sheets. Bake 9 to 11 minutes or until edges are golden brown. Let cool.