Preheat waffle iron. Preheat oven to 200 degrees F.
Whisk flour, malted-milk powder, baking powder, baking soda and salt in a mixing bowl. Whisk egg yolk, buttermilk, oil and vanilla in a medium bowl. Whisk into dry ingredients until just moistened.
Beat egg whites in a clean bowl with an electric mixer until soft peaks form. Gradually add sugar, beating until glossy but not dry. Whisk one-quarter of the beaten whites into the batter, then fold in remaining whites.
Lightly coat waffle iron with cooking spray. Spoon in batter and cook until waffle is crisp and golden. Transfer to oven to keep warm. Repeat with remaining batter, lightly coating iron with cooking spray each time, if necessary. Dust waffles with confectioners' sugar before serving, if desired.
3 other carbohydrate, 1 fat