Preheat oven to 350°F.
Butter 2 loaf pans 9 x 5 x 3 inches with 3 tablespoons of the butter.
In a mixing bowl cream together remaining butter and 1 cup of the sugar.
Beat in the egg yolks, one at a time, then stir in 1/3 cup of the lemon juice, and the grated lemon rind.
Combine cake flour with baking powder.
Add one third of the flour mixture to the creamed butter and sugar.
Then add half of the milk, another third of the flour, remaining milk and remaining flour.
Do not overmix.
In another bowl beat the egg whites and pinch of salt together until stiff but not dry.
Fold beaten egg whites and black walnuts gently into the batter.
Pour batter into the prepared pans.
Bake on the middle rack of the oven for 45 to 50 minutes, or until a cake tester inserted in the center of a loaf comes out clean.
Cool bread slightly, remove from pans, and cool completely on a rack.
Boil the remaining 1/3 cup of the lemon juice, water and the remaining 1/2 cup sugar together in a small saucepan for 2 minutes.
Drizzle the lemon syrup over the tops of the cooled loaves and let them set until completely cool before wrapping