Make the chocolate mousse: In a medium saucepan, whisk together 1 ½ cups heavy cream, egg yolks, and sugar and cook over medium-high heat, until it thickens and coats the back of a spoon, about 1-2 minutes. Whisk continuously. Remove from the heat and continue whisking for about 1-2 minutes. Add melted chocolate and whisk until combined. Transfer the chocolate custard to a large mixing bowl and let cool completely.
Preheat the oven to 350°F/175°C. Line the bottom and the sides of a 9" / 23cm baking pan with parchment paper and set aside.
Make the chocolate cake layer: In a medium-sized bowl, mix together the flour, cocoa, sugar, baking powder, and salt and set aside. In a large mixing bowl, using an electric mixer fitted with a whisk or paddle attachment, beat the oil, egg, buttermilk and vanilla, just until combined. Stir in dry ingredients to combine. Slowly mix in the hot cocoa until well combined. The batter will be thin. Pour the batter into the prepared baking pan and bake 14-18 minutes or until a toothpick inserted into the center comes out clean. Leave to cool in the pan for about 20 minutes. Then open the springform pan and let it cool down to room temperature.
Continue with the chocolate mousse: In a large mixing bowl, using an electric mixer fitted with a whisk attachment, beat the remaining 1 ½ cups heavy whipping cream until soft peaks form. Add powdered sugar and beat until stiff peaks form.
Using a whisk, stir about ¼ of the whipped cream into the cooled chocolate cream until smooth and combined. Then carefully fold the remaining whipped cream into the chocolate mixture with a spatula until combined. Don't overwork the mousse or it won't set properly.
Assemble the cake: Slice a thin layer from the top of the cooled cake layer to create a flat surface if needed. Attach the springform ring and close the pan. Then carefully pour the chocolate mousse into the pan and spread evenly. Chill in an airtight container in the refrigerator overnight.
Make the chocolate ganache: Place chopped chocolate in a large heatproof bowl and set aside. Place heavy cream in another heatproof bowl and microwave until very hot, almost boiling, about 1-2 minutes. Pour over the chocolate and let stand 1-2 minutes. Whisk until smooth and the chocolate has completely melted. Chill in the fridge for about 10-15 minutes to thicken. Test the consistency of the ganache on the side of the bowl. It should drip. The longer it cools, the thicker it gets. If it's too firm, reheat it in the microwave for a few seconds. Once it has reached the desired consistency, it can be spooned onto the cake.
Remove the ring from the springform pan and gradually spoon the ganache around the edges of the cake, letting it drip down the sides. Do this with a small spoon, making a drop at a time. Then spoon or pour the rest of the frosting onto the cake (you may not need all of it) and smooth with an offset spatula. Place the cake in the fridge for 15 minutes to allow the ganache to set. Decorate as you like. Store in an airtight container in the refrigerator for up to 3 days.