Lightly grease a 10 inch bundt or tube pan. With an electric mixer, cream the butter and 1 1/2 cups sugar, on high, for 1 minute. Beat in egg yolks. Add flour alternatively with buttermilk.
In a separate bowl, beat egg whites and 1/4 cup sugar until soft peaks form (not too stiff). Gently fold into flour mixture with poppy seeds. Spread in prepared pan. Bake at 325°F for 55 minutes or until top springs back lightly.
Cool for 45 minutes in the pan before turning out. Sprinkle with icing sugar to serve.