1. Preheat oven to 350 degrees F. Coat a 10-inch (12-cup) Bundt pan, preferably nonstick, with cooking spray and dust with flour (or use cooking spray with flour).
2. Toast poppy seeds in a small dry skillet over medium heat, stirring constantly, until fragrant, 3 to 4 minutes. Transfer to a plate to cool.
3. Whisk whole-wheat flour, all-purpose flour, baking powder, baking soda, salt and the poppy seeds in a medium bowl. Combine buttermilk, oil, vanilla, lemon zest and lemon juice in a glass measuring cup.
4. Beat eggs, egg whites and sugar in a large bowl with an electric mixer on high speed until thickened and pale, about 5 minutes.
5. Fold the dry ingredients into the egg mixture with a rubber spatula, a third at a time, alternating with 2 additions of the buttermilk mixture. Scrape the batter into the prepared pan, spreading evenly.
6. Bake the cake until the top springs back when touched lightly and a toothpick inserted in the center comes out clean, 35 to 40 minutes. Let cool in the pan on a wire rack for 5 minutes. Loosen the edges and turn the cake out onto the rack.
7. Meanwhile, prepare glaze: Sift 3/4 cup confectioners' sugar into a small bowl; mix with lemon juice and water to create a thin glaze. Poke 1-inch-deep holes all over the cake with a skewer. Coat the warm cake with the glaze using a pastry brush. Let cool completely. To serve, set the cake on a serving plate and dust with confectioners' sugar.

Yield: 16 servings