Preheat oven to 375°.
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.