For cake, preheat oven to 325 °F. Whip separated egg whites and cream of tartar until foamy. Gradually add ½ cup sugar and whip until whites hold a stiff peak and set aside. Sift remaining 1 cup sugar with pastry flour and salt. In a separate bowl, whisk egg yolks, lemon juice, lemon zest and vanilla. Add yolk mixture to flour mixture and beat with electric beaters until thick and smooth, about 2 minutes. Stir in poppyseeds. Fold in a third of the whipped egg whites until almost incorporated then fold in remaining two thirds. Scrape batter into 2 ungreased 9-x-5-inch loaf pans. Bake for 45 to 55 minutes, (you may be tempted, but don’t open the oven door for at least 35 minutes!), until a tester inserted in the center of the cake comes out clean. Immediately turn the cakes over, and let cool completely upside down (setting it in a large glass bowl works great). To remove cakes from pan, run a spatula or knife around the edge of the cakes and tap out onto a plate.
For glaze stir lemon juice into icing sugar and beat until a thick but pourable consistency. Stir in lemon zest.
To assemble, slice cakes horizontally and spread half of the glaze between the 2 cakes. Top with remaining cake layers and drip remaining glaze on top.
Lemon Ice Cream
Whisk lemon juice, zest and sugar together until sugar dissolves. Stir in buttermilk and whipping cream. Pour into ice cream maker and churn according to manufacturer’s instructions. Scrape ice cream into a non-reactive container and freeze until firm, about 2 hours.