For The Filling, Whisk Together The Melted Butter, Lemon Juice, Zest, Egg Yolks, And Sugar In A Medium-Size Bowl Until Smooth And Combined.
Pour This Mixture Into The Top Pan Of A Double Boiler, And Set It Over A Saucepan Of Simmering Water.
Cook The Mixture, Stirring, Until It Is Thick And Smooth, 35 To 40 Minutes.
Remove The Pan From The Heat, And Continue To Stir The Filling For 2 More Minutes So It Has A Chance To Cool Down.
Pour The Filling Into A Heatproof Glass Bowl, Cover With Plastic Wrap, And Chill In The Refrigerator For At Least 2 Hours.
For The Cake, Place A Rack In The Center Of The Oven, And Preheat The Oven To 325°F.
Lightly Grease And Flour Four 9″ Round Pans.
Set Aside.
(If You Don’t Have 4 Pans, You Can Stagger The Baking, Baking 2 At A Time And Cooling The Pans Between Baking, Or You Can Bake Two 9″ Layers And, Once Cooled, Split Them In Half Horizontally To Make 4 Layers.)
Sift The Flour, Baking Powder, And Salt Into A Medium-Size Bowl, And Set The Bowl Aside.
Place The Egg Yolks In A Large Mixing Bowl, And Beat With An Electric Mixer On Medium-High Speed Until Combined, About 3 Minutes.
Add The Sugar, 2 Tablespoons At A Time, And Beat Until The Mixture Is Smooth And Pale Yellow In Color.
Add The Lemon Zest And Juice, And Beat On Low Speed Until Combined.
Set Aside.
Place The Egg Whites And Cream Of Tartar In A Large Mixing Bowl, And Beat With Clean Beaters On High Speed Until Stiff Peaks Form, About 5 Minutes.
Alternately Fold The Flour Mixture And The Beaten Egg Whites Into The Egg Yolk Mixture Until Just Combined And Smooth.
Divide The Cake Between The Cake Pans, And Smooth The Tops.
Place The Pans In The Oven.
Bake The Layers Until The Cake Is Lightly Golden And Just Begins To Pull Away From The Edges, 18 To 22 Minutes If Baking 4 Layers, 25 To 30 Minutes If Baking 2 Large Layers.
Place The Pans On Wire Racks To Cool For 5 Minutes.
Run A Sharp Knife Around The Edges Of The Pans, Give The Pans A Gentle Shake To Release The Cakes, And Invert Them Once And Then Again On The Racks To Cool Completely, 30 Minutes.
To Assemble The Cake, Remove The Filling From The Refrigerator.
Place 1 Layer On A Cake Plate And Spread A Third Of The Chilled Filling Over The Top, Almost To The Sides.
Repeat With The Remaining Cake Layers, Leaving The Top Of The Cake Bare.
Place The Cake With Filling In The Refrigerator To Set While You Make The Icing.
For The Icing, Place The Butter In A Large Mixing Bowl, And Beat With An Electric Mixer On Medium-High Speed Until Creamy, 1 Minute.
Add The Egg Yolk And Beat Well.
Add The Confectioners’ Sugar In 3 Additions, Alternating It With The Lemon And Orange Juices And Zest, Beating Well After Each Addition And Until The Icing Is Creamy.
To Finish, Remove The Cake From The Refrigerator.
To Support The Cake While Frosting It, Place 3 Long Toothpicks Down Into The Cake.
Spread The Top And Sides Of The Cake Generously With Icing.
Garnish The Top With The Lemon And Orange Slices, If Desired.
Remove Toothpicks Before Slicing.
