Preheat oven to 350°F Line 20 standard (2-1/2-inch) muffin pan cups with foil baking cups.
Combine flour, 1/4 cup sugar, cocoa, baking soda, baking powder and salt in large bowl; set aside. Blend yogurt, banana, oil, milk and vanilla in small bowl; mix well.
Beat egg whites in small deep bowl at medium speed with electric mixer until foamy. Gradually add remaining 1/2 cup sugar, beating well after each addition, until sugar is dissolved and stiff peaks form. Stir yogurt mixture into flour mixture just until dry ingredients are moistened. Gently fold in 1/3 of egg white mixture until blended; fold in remaining egg white mixture. Spoon batter into prepared muffin cups, filling 2/3 full.
Bake 20 to 25 minutes or until toothpick inserted into centers comes out clean. Transfer cupcakes from pans to wire racks; cool completely. Meanwhile, powdered Sugar Glaze; drizzle over cupcakes; let stand until set. Store in airtight container at room temperature.
Powdered Sugar Glaze Blend 1/2 cup powdered sugar and 1 tablespoon water in small bowl until smooth; add additional water, if necessary, to reach desired consistency.