Heat oven to 350°F.
Line muffin pans with paper or foil-laminated paper baking cups (2 1/2 inches in diameter).
In large bowl, stir together flour, 1/4 cup sugar, cocoa, baking soda, baking powder and salt; set aside.
In small bowl, stir together yogurt, banana, oil, milk and vanilla; set aside.
In medium mixer bowl, beat egg whites until soft peaks form.
Gradually beat in remaining 1/2 cup sugar; beat until stiff peaks form.
Stir yogurt mixture into flour mixture until dry ingredients are moistened; fold in 1/3 of egg white mixture.
Gently fold in remaining egg white mixture.
Fill muffin cups 3/4 full.
Bake 20 to 25 minutes or until wooden toothpick inserted in centers comes out clean.
Remove from muffin pans to wire racks; cool completely.
Drizzle tops of cupcakes with White Glaze.
White Glaze: In small bowl, combine 1/2 cup powdered sugar with 3 to 4 teaspoons warm water; stir until smooth and of desired consistency.