Preheat oven to 325 degrees F. Grease and flour a loaf pan or use a 6 inch Bundt pan.
With a handheld mixer, beat the egg whites until soft peaks form. Set them aside.
Using the same mixer, beat the butter and sugar in a mixing bowl until light and creamy.
Stir together the sifted cake flour, baking soda and salt. If using poppy seeds, stir them in as well. Gradually add to the batter, mixing on low speed (or stirring by hand) until blended. Add egg yolks and stir until mixed, then add vanilla extract, lemon extract and lemon juice and stir well. Stir in the sour cream.
Fold in the beaten egg whites.
Pour into pan and bake and 325 for 45-50 minutes or until cake tests done.
To make the icing, put the sugar in a large bowl. Add lemon juice 1 tablespoon at a time, stirring all the while, until the consistency is somewhat like glue. Pour or drizzle over cooled cake.