Place A Rack In The Center Of The Oven, And Preheat The Oven To 350°F.
Lightly Grease A 9″ Loaf Pan With Butter And Dust With Flour.
Shake Out The Extra Flour, And Set The Pan Aside.
Place The Chopped Black Walnuts On A Baking Pan And Place In The Oven To Toast While The Oven Preheats.
Let Them Toast For 5 To 7 Minutes, And Let The Nuts Cool On The Pan.
Place The Butter And Brown Sugar In A Large Mixing Bowl, And Blend With An Electric Mixer On Medium-High Speed Until Fluffy And Pale Yellow In Color, About 3 Minutes.
Add The Egg Yolks, One At A Time, Beating Well After Each Addition, Scraping The Bowl As Needed.
Set This Batter Aside.
Sift The Flour And Baking Powder Into A Small Bowl.
Fold A Third Of The Flour Mixture Into The Reserved Batter With A Rubber Spatula, And Add Half Of The Water.
Add Another Third Of The Flour Mixture, And Then The Rest Of The Water, And The Final Third Of The Flour.
Toss The Toasted Nuts With The Reserved 1 Teaspoon Flour, And Fold These Into The Batter.
In A Medium-Size Bowl, Beat The Egg Whites And Salt With An Electric Mixer On High Speed Until Stiff Peaks Form, 3 To 4 Minutes.
Fold The Egg Whites Into The Batter, Just Until Combined, Being Careful To Incorporate The Egg Whites But Not Overbeat Them.
Pour The Batter Into The Prepared Pan, Smoothing The Top With The Spatula, And Place The Pan In The Oven.
Bake The Cake Until The Top Is Golden Brown And A Toothpick Inserted In The Center Comes Out Clean, 42 To 47 Minutes.
Place The Loaf Pan On A Wire Rack To Cool For 10 Minutes, Then Invert The Cake Once And Then Again To Cool Right Side Up On The Rack To Room Temperature, 1 Hour.
Slice And Serve.
