Preheat oven to 180 degrees C. Lightly grease an 20cm square baking pan.
Place the raisins and rum in a microwave-safe bowl. Place in the microwave and cook on High for 1 minute. Set aside until needed.
Stir the flour, baking powder, bicarb soda and salt together in a mixing bowl. In another mixing bowl, beat the butter and brown sugar together until light and fluffy. Beat in the banana, buttermilk, vanilla and egg whites until well blended. Gradually add the flour mixture, beating just until combined. Stir in the raisin mixture and walnuts. Spread the batter into the prepared pan.
Bake in preheated oven until the top is golden, about 30 minutes. Cool in the pan 10 minutes before cutting into bars and transferring to a rack to cool completely. Dust with powdered sugar.