For The Filling, Combine The Egg Yolks, Confectioners’ Sugar, And Milk In A Large Mixing Bowl, And Blend With An Electric Mixer On Low Speed Until Combined, 1 Minute.
Pour The Mixture Into The Top Of A Double Boiler.
Bring 1″ To 2″ Water In The Bottom Of The Double Boiler To A Boil, Then Reduce The Heat To Medium.
Place The Top Of The Double Boiler Over The Water.
Whisk And Let Cook Until The Custard Has Thickened, 8 To 10 Minutes.
Pour Into A Clean Large Bowl, And Stir In The Vanilla.
Set The Bowl Aside.
Place The Butter And Salt In A Medium-Size Mixing Bowl, And Blend With The Mixer On Low Speed Until Well Creamed, 1 Minute.
Slowly Pour The Cooled Custard Mixture Into The Butter, And Blend On Low Speed Until Combined And Smooth.
Set Aside.
For The Cake, Place The Egg Yolks, Water, And ½ Cup Of The Sugar In A Large Mixing Bowl, And Beat With The Electric Mixer On Medium Speed Until Light, About 2 Minutes.
Add The Flour Gradually, In 4 Additions, Beating For About 20 Seconds On Medium Speed For Each Addition.
Blend In The Vanilla And Set Aside.
Place A Rack In The Center Of The Oven, And Preheat The Oven To 350°F.
Set Aside An Ungreased 10″ Tube Pan.
Place The Egg Whites, Cream Of Tartar, And Salt In A Large Mixing Bowl, And Beat With Clean Beaters On High Speed Until Soft Peaks Form, 2 To 3 Minutes.
Gradually Add The Remaining ½ Cup Sugar And Beat Until Stiff Peaks Form, 2 Minutes More.
Fold The Egg Whites Into The Batter Until Just Combined.
Pour The Batter Into The Prepared Pan, And Place The Pan In The Oven.
Bake The Cake Until It Is Lightly Browned And Springs Back When Lightly Pressed, 32 To 38 Minutes.
Remove The Pan From The Oven, And Place It On A Wire Rack To Cool Upside Down In The Pan.
Meanwhile, Make The Icing.
Place The Sugar And Water In A Medium-Size Saucepan, And Bring To A Boil, Stirring With A Wooden Spoon.
Cook On Medium-Low Heat Until The Sugar Melts And The Syrup Spins A Thread When You Drop It From The Spoon.
It Needs To Reach 220°F On A Candy Thermometer, Which Takes 6 To 8 Minutes.
While The Sugar Is Cooking, Beat The Egg Whites And Salt In An Electric Mixer On High Speed Until Soft Peaks Form, 2 To 3 Minutes.
Very Slowly Drizzle The Hot Sugar Syrup Into The Beaten Egg Whites At Medium Speed.
Beat Constantly Until The Frosting Stands In Stiff Peaks And Is Of Spreading Consistency.
Add The Vanilla And Beat Well For 20 Seconds More.
To Assemble, Run A Knife Around The Edges Of The Cake Pan, Give The Pan A Good Shake, And Invert The Cake Onto The Rack.
Slice The Cake Horizontally Into Thirds.
Carefully Remove The Top 2 Layers And Set Them Aside.
Place The Bottom Layer On A Cake Plate.
Spread Half Of The Filling Over This Layer, Just To The Edges.
Place The Middle Layer On Top.
Spread The Remaining Filling Over It, Just To The Edges.
Place The Top Cake Layer Over The Filling.
Generously Frost The Sides And Top Of The Cake, Including Down Into The Hole In The Center.
Garnish The Top With The Orange Zest.
Slice And Serve.
