For The Frosting, Place The Milk, Sugar, Melted Butter, And Egg Yolks In A Large Mixing Bowl, And Whisk To Combine.
Pour The Mixture Into A Medium-Size Heavy Saucepan, And Place Over Medium Heat.
Cook, Whisking Vigorously, Until The Mixture Comes To A Boil.
Boil For 1 Minute.
Remove The Frosting From The Heat, And Whisk For 1 Minute To Cool It Down.
Whisk In The Vanilla.
Pour Into A Heatproof Bowl, Cover The Top Of The Frosting With Plastic Wrap, And Place The Bowl In The Refrigerator To Chill For 4 Hours.
For The Filling, Place The Sugar, Cornstarch, And Salt In A Medium-Size Heavy Saucepan And Stir To Combine.
Add The Egg And Milk, And Stir To Combine.
Place The Pan Over Medium Heat And Cook, Stirring Until The Mixture Comes To A Boil, And Then Thickens, 3 To 4 Minutes.
Take The Pan Off The Heat, And Stir In The Butter And Vanilla.
Pour Into A Heatproof Bowl, Cover The Top Of The Filling With Plastic Wrap, And Place The Bowl In The Refrigerator To Chill For 3 Hours.
For The Macadamia Nut Garnish, Place A Rack In The Center Of The Oven, And Preheat The Oven To 350°F.
Place The Macadamia Nuts In A Small Baking Pan, And Place In The Preheating Oven.
Toast Until Golden Brown, 4 To 5 Minutes, Watching Carefully So The Nuts Do Not Burn.
Remove The Pan From The Oven, And Set Aside.
For The Cake, Lightly Grease And Flour Two 9″ Round Cake Pans, Shake Out The Excess Flour, And Set The Pans Aside.
Separate The Eggs, Placing The Whites In A Large Mixing Bowl And The Egg Yolks In A Smaller One.
Beat The Egg Whites With An Electric Mixer On High Speed Until Foamy, 1 To 2 Minutes.
Gradually Add ½ Cup Of The Sugar, Continuing To Beat On High Until Stiff Peaks Form, 1 To 2 Minutes More.
Set The Egg Whites Aside.
Sift The Remaining 1 Cup Sugar And The Flour, Baking Soda, And Salt Into A Large Mixing Bowl.
Add The Melted Chocolate, Oil, And ½ Cup Of The Milk.
Blend On Low Speed Until Just Combined.
Add The Egg Yolks And Remaining Milk, And Beat On Medium Speed Until Smooth, 1 Minute.
Fold In The Egg Whites Until No Traces Of White Remain.
Divide The Batter Between The Prepared Pans, And Place The Pans In The Oven.
Bake The Cakes Until They Spring Back When Lightly Pressed In The Center, 22 To 26 Minutes.
Remove The Pans From The Oven And Let Cool 10 Minutes.
Run A Knife Around The Edges Of The Pans, Give The Cakes A Gentle Shake, And Then Invert Once And Then Again Onto A Wire Rack To Cool Completely, 1 Hour.
Slice Each Layer In Half Horizontally.
To Assemble The Cake, Remove The Filling And Frosting From The Refrigerator.
Place The Bottom Half Of 1 Layer On A Cake Plate With The Cut Side Up.
Divide The Filling Into Thirds, And Spread 1 Part Onto The Top Of The Cake.
Place The Top Of The Layer Over The Filling, And Spread With Another Third Of The Filling.
Place The Bottom Of The Second Layer On Top Of The Filling, And Spread With The Remaining Filling.
Place The Top Of The Second Layer Over The Filling.
Place The Cake In The Refrigerator For 15 Minutes To Make The Cake Easier To Frost.
Remove The Cake From The Refrigerator, And Spread A Thin Layer Of Frosting Around The Edges Of The Cake To Seal The Crumbs.
Place The Cake Back In The Refrigerator To Set The Frosting, About 30 Minutes.
Remove The Cake From The Refrigerator, And Spread The Remaining Frosting On The Top And Sides Of The Cake.
Garnish With The Toasted Macadamia Nuts.
