Step 1

Preheat oven to 350 degrees. Line bottom of a 9"x13" metal pan with parchment paper. Trim edges to fit bottom.

Step 2

In a small bowl, blend cocoa with boiling water until smooth. Set aside.

Step 3

Place egg whites into the bowl of a standing mixer. Mix on medium speed. When they become a little frothy, add the cream of tartar. Continue on medium speed. Increase to high speed when the volume has doubled. Slowly add in the ½ c. sugar, while keeping the mixer on. Continue whipping until soft peaks that can hold their shape form (this should only take a minute or so). Place the whipped egg whites into a large mixing bowl, pushing them to one side of the bowl.

Step 4

Place the cocoa-water mixture into the standing mixer bowl (no need to wash the bowl first). Add butter, oil, eggs, and vanilla.

Step 5

In a separate mixing bowl, whisk together flour, 1 c. sugar, baking powder, baking soda, and salt. Add to the cocoa mixture. Mix on medium low speed for 1 minute until blended, then increase to medium speed for 3 minutes.

Step 6

Pour batter into the open side of the bowl with the whipped egg whites, so that the batter flows mostly to the bottom and below the egg whites.

Step 7

Fold the egg whites into the cake batter. Place a spatula down one side of the bowl to the bottom, push chocolate batter from underneath to the other side of the bowl, then lift spatula up the other side of the bowl and over the top, carrying some chocolate batter with it. Continue this pattern, repeating it while turning the bowl ¼ turn at a time, and seeing the streaks of chocolate batter and egg whites slowly incorporate together.

Step 8

The final chiffon cake batter should be uniform in color, without any streaks of white (those make tough areas in the cake). If necessary, use the spatula to gently spread through the white streaks to help blend them into the chiffon cake batter. Whatever you do, do not stick the spatula into the bowl and stir vigorously in circles--that would deflate the air bubbles and yield a dense cake.

Step 9

Pour batter into the prepared cake pan. Bake for 33-35 minutes, or until toothpick inserted into the center comes out clean. Invert onto a cake rack and cool completely.

Step 10

Melt butter in a medium sized saucepan over medium low heat. Remove from heat and cool slightly. Whisk in milk, sugar, and vanilla first, then eggs and egg yolks one at a time.

Step 11

Cook on medium heat to a low boil, whisking intermittently.Once it boils, let it bubble and cook for another 2 minutes, continuing to whisk intermittently. Remove from heat and quickly sift cornstarch over the frosting while whisking it in. Whisk quickly and forcefully to prevent lumps from forming.

Step 12

Pour the frosting into a shallow dish such as a glass pyrex pie pan. Cool to room temperature. Cover and chill about 3 hours, until it solidifies to a spreading consistency.

Step 13

Make the simple syrup in a small pot by adding the sugar to the water and bringing to a boil. Set aside.

Step 14

Loosen the cake edges from the sides of the pan with a butter knife. Place a cake platter over the top of the pan and flip everything over, unmolding the cake onto the platter. Peel off the parchment paper from the cake. Use a large serrated knife to split the cake into 2 layers. Insert a flexible cutting mat between the 2 layers to lift off the top layer. Invert this top layer onto another mat so that both layers are cut side up. Brush each layer with half of the simple syrup.

Step 15

Mix chilled frosting smooth with a rubber spatula. Place one third of the frosting onto the bottom cake layer. Use a small, metal angled spatula to spread frosting evenly out to the edges. Place the top layer, cut side down, onto the frosted bottom layer.

Step 16

Working a small section at a time, place some frosting onto the cake top, then push it over the edge and quickly frost the side. Repeat until all the sides are covered. Frost the top last. Finally, sprinkle the macadamia nuts over the top.

Step 17

Cover and chill at least 24 to 48 hours before serving. Keep refrigerated.