Preheat oven to 350°F / 180°C and prepare an 8-inch cake pan.
Put all ingredients (except hot water) in a large bowl and mix together until totally incorporated and free from lumps. I use a stand mixer but you can also do this by hand.
Add the hot water into the batter and stir until combined.
Pour into prepared cake pan and bake for 30 - 40 minutes or until a skewer comes out clean.
Let cool completely, remove from the cake pan, then slice off the rounded top so you are left with one thin cake base layer.
Clean and dry the cake pan, then place the cake base layer on the bottom and set aside while you prepare the other layers.
Make the Chocolate Cheesecake
Melt the dark chocolate in the microwave in 30-second increments until totally melted. Set aside to cool slightly.
Cream the cream cheese, powdered sugar and vanilla together until completely smooth and free from lumps.
Pour melted chocolate into the cream cheese mix and beat until combined.
In a separate bowl, whip up the whipping cream until stiff peaks form.
Take a small amount of the whipped cream and mix it into the chocolate cream cheese mixture to loosen it up a little. Then fold in the remaining cream.
Pour the chocolate cheesecake into the cake pan, on top of the cake layer, then put in the fridge to chill while you prepare the white chocolate mousse layer.
Make the White Chocolate Mousse
Melt the white chocolate in the microwave in 30-second increments.
While it cools slightly, whip up the whipping cream and vanilla until stiff peaks form.
Pour the melted white chocolate into the whipped cream and fold gently until completely combined.
Pour into the cake pan on top of the chocolate cheesecake layer and put in the fridge to set for at least 3 hours.
Make the Chocolate Ganache
Once the mousse cake has set, break up your milk and dark chocolate and put them into a bowl.*
Pour the cream into a saucepan and heat on medium-low until it starts to steam and tiny bubbles begin appearing on the surface.
Remove the cream from the heat and pour it over the chocolate. Let sit for a minute and then gently start to stir. Keep going until the chocolate is totally melted and it turns into a smooth, glossy mixture.
Let the ganache cool slightly, the perfect pouring temperature is about 89°F / 32°C. If you don't have a thermometer just look for the ganache to thicken slightly so it leaves a slight trace in itself when you drizzle it back into the bowl.
Decorate the Black Tie Mousse Cake
Once the mousse cake is set and the ganache is at the right temperature, carefully remove the cake from the cake pan.
Reserve about a quarter of the ganache and set it aside to cool further for piping later.
Place the cake on a wire rack and pour the chocolate ganache all over the cake.
If you want to add the design on top, melt a small about of white chocolate and pipe a design on top of the ganache immediately after pouring.
While the ganache is still wet, use your hands to press mini chocolate chips or chocolate flakes into the sides of the cake.
Once the leftover ganache has cooled to piping consistency, put it in a piping bag and pipe around the outside of the mousse cake.
Notes
Cake pan - you have a few options for what kind of round pans you can use to build the Black Tie Mousse Cake in - you can use a loose-bottomed cake pan, a springform pan or a tart ring (the ones with no bottom). As long as it will support the sides of the cake while they set and you can remove the sides easily - it will work. The important thing is that whatever you build the mousse cake in is the same size or smaller than the cake pan you use to bake the chocolate cake in. I used a loose-bottomed 8 inch cake pan to bake the cake and then after removing the chocolate cake I cleaned the pan and used the same one to build the mousse cake. Also pay attention to the height of your pan - it needs to be tall enough to house all the layers comfortably. If it isn't, you can use some acetate around the inside of the pan to make it taller.For the chocolate ganache I used a mixture of milk chocolate and dark chocolate. If you don't want to do this you can pick one or the other, or use semi-sweet chocolate, for the total amount of 420g chocolate.