Preheat waffle iron. Preheat oven to 200 degrees F.
Toast poppy seeds in a small dry skillet over medium heat until fragrant, 30 to 60 seconds. Transfer to a mixing bowl and whisk in flour, baking powder, baking soda and salt.
Whisk egg whites, egg, yogurt, sugar, oil, vanilla and lemon zest in a medium bowl. Whisk into dry ingredients until just moistened.
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 nonstick spray each time, if necessary. Dust the waffles with confectioners' sugar before serving, if desired.
3 other carbohydrate, 1 fat