Pre-heat oven to 350F. Butter and flour a 10-inch tube pan, plain or fluted.

Sift flour, baking powder, baking soda, and salt onto a sheet of waxed paper or into a bowl. Set aside.

Beat butter in mixing bow on moderately high speed until light and fluffy, about 3 minutes.

Add granulated sugar in two additions, beating for 2 minutes after each addition.

Add eggs yolks, one at a time, beating on high for 1 minute after each addition and scraping down the sides of bowl often with rubber spatula.

Beat in lemon zest and vanilla.

On low speed, beat in flour mixture in three additions and buttermilk in two, beginning and ending with the flour mixture.

Stir in poppy seeds

Beat egg whites in a clean mixing bowl with clean beaters until foamy. Add pinch of salt; continue beating until just firm, not stiff, peaks form.

Stir about 1/4 of the beaten egg whites into batter to lighten; fold in remaining whites carefully, but thoroughly.

Pour batter into prepared pan; bake in lower third of oven until wooden pick inserted into cake is withdrawn clean and dry, about 55 minutes.

Cool cake in pan 10 minutes. Turn out of pan onto wire rack to cool completely; if plain tube pan was used, invert cake onto second rack to cool right-side up.

At serving time, sprinkle top of cake with confectioners sugar.