Preheat oven to 350. Generously butter dozen cup Bundt pan. Combine half cup sugar and cinnamon in small bowl. Sift flour, baking powder, baking soda and salt into medium bowl.
Using electric mixer, beat butter and vanilla in large bowl until fluffy. Gradually add 1 cup sugar, beating until mixture is smooth. Beat in egg yolks 1 at a time. Mix in flour mixture alternately with buttermilk in 3 additions each. Fold in poppy seeds.
In medium bowl, whisk egg whites until foamy. Gradually add remaining half cup sugar, beating. Fold whites into batter in 2 additions.
Spoon half of cake batter into prepared pan. Sprinkle with half of cinnamon sugar. Spoon remaining batter over. Sprinkle with remaining cinnamon sugar.
Bake cake until tester inserted near center comes out clean, about 50 minutes. Cool cake in pan on rack 10 minutes. Turn cake out onto rack and cool completely.