Combine the raisins & rum in a microwave safe bowl. Microwave at high for 1 minute, and set aside.
Combine the flour, powder, soda & salt in a bowl, stirring well with a whisk; set aside.
Combine brown sugar & butter in a large mixing bowl, beat at medium speed until well-blended. Add banana, buttermilk, vanilla & egg whites, beating well. Mix in the dry ingredients just until blended; stir in raisin mixture & walnuts.
Spread the batter into an 8" 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. Yield: 16 bars
