For The Custard, Place The Sugar, Salt, Flour, Cornstarch, And Cocoa In A Medium-Size Bowl, And Whisk To Combine.
Set Aside.
Place The Eggs In Another Medium-Size Bowl, And Lightly Beat Until Lemon Colored.
Set Aside.
Place The Butter And Chocolate In A Large Saucepan Over Low Heat.
Stir To Melt The Butter And Chocolate.
Add The Milk And Beaten Eggs, And Whisk Until Combined.
Add The Dry Ingredients, And Whisk To Combine.
Increase The Heat To Medium And Cook Until The Mixture Is Very Thick, Whisking Constantly, 12 To 15 Minutes.
Remove From The Heat, Then Whisk In The Vanilla.
Pour The Hot Custard Into A Large Heatproof Bowl.
Cover The Surface Of The Custard With Plastic Wrap, Sealing To The Edges.
This Prevents The Custard From Developing A Tough Surface As It Cools.
Cool On The Counter For 30 Minutes, Then Place In The Refrigerator Until Cold.
This Takes As Little As 4 Hours, Or Overnight.
For The Cake, Place A Rack In The Center Of The Oven, And Preheat The Oven To 375°F.
Grease And Flour The Bottom Of As Many 9″ Layer Pans As You Have.
You Will Be Baking 8 Layers.
Shake Out The Excess Flour, And Set The Pans Aside.
Place The Butter, Sugar, And Salt In A Large Mixing Bowl, And Beat With An Electric Mixer On Medium Speed Until Light And Creamy, 1 To 2 Minutes.
Add The Egg Yolks, One At A Time, Beating Well After Each Addition.
Set Aside.
Place The Egg Whites In The Large Bowl Of An Electric Mixer, And Beat On High Speed Until Stiff Peaks Form, About 5 Minutes.
Set Aside.
Sift Together The Flour And Baking Powder In A Medium-Size Bowl.
Add The Flour Mixture To The Butter Mixture Alternately With The Milk, Beginning And Ending With The Flour.
Fold In The Vanilla And Lemon Juice.
Fold In The Egg Whites Until Just Combined.
Divide The Batter Into Eighths—About 6 Ounces Batter Or A Full ¾ Cup Per Pan.
Spread This Into Each Cake Pan, Using A Metal Spatula To Spread The Batter To The Edges.
Bake 2 To 3 Layers At A Time, Depending On The Size Of Your Oven.
Bake Until The Layers Are Lightly Golden Brown And The Edges Pull Away From The Sides, 10 To 12 Minutes.
Remove To Wire Racks, Cool For 5 Minutes, Then Run A Knife Around The Edges And Invert Onto The Racks To Cool.
The Layers Will Cool Quickly Because They Are Thin.
Wipe Out The Pans, Grease And Flour Again, And Repeat With The Remaining Batter.
To Begin Assembly, Place 1 Layer On A Serving Plate.
Dollop About 8½ Ounces Of Custard (About ⅞ Cup) On Top, And Spread It Out Evenly, To Within ½” Of The Edges.
Continue Stacking Layers And Custard.
There Will Be 7 Layers Of Custard, With A Top Cake Layer Bare.
Lightly Cover The Cake, And Place It In The Refrigerator To Firm Up The Custard, About 1 Hour.
Meanwhile, Prepare The Frosting.
Place The Confectioners’ Sugar And Butter In A Large Mixing Bowl, And Beat With An Electric Mixer On Low Speed For 30 Seconds.
Increase The Speed To Medium And Beat Until Creamy, 2 To 3 Minutes.
Add The Cocoa, Melted Chocolate, And Vanilla, And Beat Until Smooth, 1 Minute.
If The Frosting Is Too Thick, Add A Little Hot Water, 1 Teaspoon At A Time, Very Slowly Until The Consistency Is Right.
Remove The Cake From The Refrigerator.
Frost The Sides Of The Cake First, Spreading On A Thin Layer.
Frost The Top, Spreading A Thin Layer To The Edges.
Store The Cake In A Cake Saver In The Refrigerator For Up To 3 Days.
Slice And Serve.
