Let egg whites stand at room temperature for 30 minutes. In a large bowl, combine the flour, sugar, baking powder and salt. In a small bowl, whisk the egg yolks, water, oil, lemon extract, lemon zest and poppy seed filling. Add to dry ingredients; beat until well blended. In another bowl and with clean beaters, beat egg whites and cream of tartar on high speed until stiff peaks form. Fold into egg yolk mixture.
Gently spoon batter into an ungreased 10-in. tube pan. Cut through the batter with a knife to remove air pockets. Bake on the lowest rack at 350° for 55-60 minutes or until top springs back when lightly touched. Immediately invert pan; cool completely. Run a knife around edge and center tube of pan. Invert cake onto a serving plate.
For frosting, beat butter and confectioners' sugar in a large bowl. Add the lemon juice, extract and enough milk to achieve desired consistency. Frost cake.