Preheat oven to 350 degrees F. Grease and flour a bundt pan.
Cream together butter and 1 1/4 cups of the granulated sugar. Stir in 1/4 cup milk and vanilla.
Sift together flour, salt, and baking powder. Stir in poppy seeds. Set aside.
Alternately add the flour mixture and the 3/4 cup milk, a little bit at a time, stirring gently after each addition, starting and ending with flour.
Beat the egg whites in a large bowl until they stand in soft peaks and then beat in remaining 1/2 cup sugar. Fold egg whites into cake batter.
Pour batter into greased bundt pan. Bake for 35-40 minutes or until a toothpick inserted in the center comes out clean. Allow to cool for a few minutes in the bundt pan before inverting on a wire rack.
Sift powdered sugar over the top of the cake before serving.
Egg Whites: Real egg whites are essential for this cake and can't be substituted. For best results, separate the egg whites from the yolks when the eggs are cold. Then, bring the egg whites to room temperature before whipping, as they will have better whipped volume at room temperature. Freezing Instructions: Once cooled, the cake can be covered tightly and frozen for up to 3 months. Thaw overnight on the countertop before serving.