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1
Preheat oven to 350 F.
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2
Grease and flour a 10-inch (2 L) tube pan, plain or fluted.
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3
Sift flour, baking powder, baking soda and salt onto a sheet of waxed paper and resift twice more.
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4
Beat butter in mixer bowl on moderately high speed until light and fluffy, about 3 minutes.
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5
Add granulated sugar in two additions, beating for 2 minutes after each addition.
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6
Add egg yolks, one at a time, beating on high speed for 1 minute after each addition and scraping down sides of bowl often with rubber spatula.
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7
Beat in lemon zest and vanilla.
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8
On low speed, beat in flour mixture in three additions and buttermilk in two, beginning and ending with flour mixture.
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9
Stir in poppy seeds.
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10
Beat egg whites in clean mixer bowl with clean beaters until foamy.
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11
Add pinch of salt; continue beating just until firm, not stiff, peaks form.
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12
Stir about one fourth of the beaten whites into batter to lighten; fold in remaining whites carefully but thoroughly.
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13
Pour batter into prepared pan; bake in lower third of oven until wooden pick inserted into cake is withdrawn clean and dry, about 55 minutes.
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14
Cool cake in pan 10 minutes.
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15
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.
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16
At serving time, sprinkle top of cake with confectioners' sugar.
