Preheat the oven to 325 degrees F. Spray your bundt pan (I use this one) with nonstick spray, coat with flour and set aside.
In a medium size mixing bowl, combine the flour, baking powder, salt, lemon zest and poppy seeds. Whisk to combine. Set aside.
In the bowl of an electric mixer, fitted with the paddle attachment, cream the butter and the sugar together on medium speed for about 2 to 3 minutes.
Add the egg whites, in three additions, making sure to mix well between each addition and scraping down the sides of the bowl.
Add the sour cream and mix.
With the mixer on low speed alternately add the flour mixture and the lemon juice to the butter mixture, starting and finishing with the flour mixture. Mix until combined.
Pour the batter into your prepared bundt pan. Bake for about 45 to 50 minutes, or until a toothpick inserted in the center of the cake comes out with a few moist crumbs on it.
Let the cake cool in the bundt pan for at least 30 minutes before inverting onto a wire rack. Trying to remove the cake from the pan too quickly is usually what causes the cake to stick and fall apart. I left my cake in the pan for an hour before flipping it.