Place banana in a food processor; process until smooth.
Add margarine, rum, vanilla, and egg whites; process until blended.
Combine 3/4 cup plus 2 tablespoons powdered sugar, flour, baking powder, and salt in a bowl, and add to food processor. Pulse 2 to 3 times or until combined.
Add nuts; pulse 2 times or until well-blended.
Spoon 1 tablespoon batter into each of 12 madeleine molds coated with baking spray.
Bake at 375 for 15 minutes or until puffy.
Remove from pan immediately; let cool on a wire rack. Repeat procedure with remaining batter. Sift 2 tablespoons powdered sugar over madeleines.