Preheat the oven at 160C (fan assisted) or 320 Fahrenheit. Prepare your mould, I used a silicon loaf pan but you can really use any form that's around 10.4 x 5.5 inches long and wide and has a capacity of 1000ml or 33.8 Fluid Ounces.
In a medium sized bowl, combine the flour, cocoa powder and baking powder. Whisk and set aside until needed.
In a medium sized heat proof bowl, combine 2 Eggs and 1 Egg Yolk from the remaining Eggs along with half of the sugar and the vanilla extract. lightly whisk together. Add the softened butter.Add hot water (60-70c) to a large sauce pan, please the bowl with the egg, sugar and butter into the sauce pan. Now whisk with an electric mixer until fluffy and pale. This should take around 6-7 minutes.
Sift half of the dry ingredient mix into the fluffy egg/sugar/butter mix and fold with a spatula until combined. Now add the remaining mix and fold until combined. Set aside until needed.
Combine the egg white from the remaining eggs in a large bowl and whisk with an electric mixer until lightly fluffy. Now add 1/2 of the remaining sugar and whisk for 30-40 seconds until stiff peaks form. Add the remaining sugar and whisk for another 30-40 seconds. Now fold the meringue into the cocoa batter one third at a time until well combined and smooth. Pour into the mould and bake for 20-30 minutes at 170C or 330F or until a wooden skewer test comes out clean.
4 Large Egg, 110 gram Crystal Sugar
White Chocolate Mousse
In a large bowl, whisk the heavy cream until medium stiff. It has to still be runny so don't whisk all the way. Divide into two equal portions and store in the fridge until needed.Now put half of the gelatin in a cup of ice cold water and let it rest for 10 minutes or until needed. ( you will need to do this again after with the remaining gelatin)
200 gram Heavy Cream, 4.5 gram Gelatin
Note, this step is performed with half of the ingredients, you need to do this twice, once to fill the half of the mould, then add the cake layer and then add the 2nd half of the mousse. Place a small heatproof bowl into a sauce pan with hot water, note that this is NOT a Bain Marie, so the bowl CAN touch the water. Add the chocolate and stir until almost dissolved. When only a few pieces of chocolate are left, remove it from the sauce pan and stir until completely dissolved. Microwave half of the milk to 80c, drain the water out of the gelatin and place it into the hot milk. Stir a bit with a spoon to fully dissolve it. Now add it into the chocolate and stir until smooth and combined. Whisk in half of the heavy cream until smooth and combined.
200 gram White Chocolate, 4.5 gram Gelatin, 120 gram Full Fat Milk
Pour the white chocolate mousse mixture into the silicon mould and place it into the freezer for 20-30 minutes. It doesn't need to freeze solid at this stage but must be strong enough to support the cake layer.
While the mousse is setting, trim the cake edges and slice it into two equal layers. Try and get them as even as possible. The kids can have the trimmings :)
Now repeat the stem with the white chocolate and heavy cream with the remaining ingredients. Its now time to assemble the mousse. Place the middle cake layer into the form and pour the remaining mousse over it to fill the rest of the form. Shake it a little to remove any bubbles but be careful not to move the cake layer. Now add the final cake layer which will act as the bottom of the cake.Place it in the freezer for no less than 6 hours. Removing it any sooner than that will leave marks on the cake or even break it when de-moulding.
Mirror Glaze
In a medium sauce pan, combine the water, sugar and cocoa powder. Stir until smooth and fully dissolved (this might take a bit since cocoa powder is hydrophobic) Now add in the Corn Syrup and bring the mixture to a boil. Let it boil for no more than 20-30 seconds and take it off the heat. Pour into a heatproof container through a sieve to remove any bubbles and cocoa clumps. Now let it cool until it reaches 28c or 85F.
To pour the glaze over the mousse cake evenly, the best way to do this is over a roasting pan and prepping it up with a couple of cups or glasses. Use a palette knife to clean off the edges and let it set.