Preheat oven to 350°.
Combine the raisins and rum in a microwave-safe bowl. Microwave at high for 1 minute, and set aside. Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and the next 3 ingredients (flour through salt) in a bowl, stirring well with a whisk. Set aside.
Combine brown sugar and butter in a large bowl, and beat with a mixer at medium speed until well blended. Add banana and next 3 ingredients (banana through egg whites), beating well. Add the flour mixture, beating just until combined. Stir in raisin mixture and walnuts.
Spread the batter into an 8-inch square baking pan coated with cooking spray. Bake at 350° for 30 minutes or until golden. Cool the bars completely on a wire rack. Sprinkle with powdered sugar.