Chocolate Genoise Sponge Cake:

1. Pre-heat the oven to 350F and line a 9" cake pan with parchment paper on the bottom; don't grease the sides of the pan
2. Whisk the eggs and sugar using an electric or stand mixer for 3-4 minutes until they triple in volume, become light in color and fall off the whisk in a ribbon. Do not rush this step
3. Sift in the flour, cocoa powder, salt and baking powder, and use a rubber spatula to fold very gently and slowly until they are just combined. Take care not to knock out the air in the batter, and do not overmix as that can cause the cake to become dense
4. Pour the batter into the pan, shake to spread it out, and bake for ~12-14 minutes until a toothpick inserted in the middle comes out with light, moist crumbs (almost clean, no wet batter)
5. Let the cake cool in the pan for 10 minutes, then carefully turn it out and place on a wire rack. In the meanwhile, prepare the mousse

Chocolate mousse:

1. First, make a ganache with the chocolate and 1 cup (240g) heavy cream. Place the chopped chocolate in a bowl. Heat the cream on the stove until it just starts to simmer, and pour over the chocolate. Cover with a lid or plate and wait 3-5 minutes to allow the chocolate to melt. Then stir gently until you get a creamy smooth ganache. Let the ganache rest for 5 minutes
2. Add the remaining 1 1/2 cup (360g) chilled cream to a large bowl. Use an electric hand mixer to whisk the cream for a few minutes until it reaches medium peaks and holds shape - take care not to overmix
3. Add about a third of the whipped cream from step 2 into the ganache from step 1 (which should not be hot at this point), and use a rubber spatula or whisk to very gently and very slowly fold it in. Be gentle, and stop when they are mostly combined
4. Add the whole mixture into the remaining whipped cream, and continue to gently and slowly fold. If you see any lumps of whipped cream, press down on them with your spatula to break them. Stop mixing as soon as the cream has fully mixed with the ganache. You should have a smooth and fluffy chocolate mousse

Assembly:

1. Take a 9" springform pan and line it with a parchment circle on the bottom and a strips of parchment on the sides, using a little butter or oil to stick the parchment to the pan (see images above). This will allow unmolding the cake easily
2. Place the cooled cake inside the pan, and then pour over the chocolate mousse. Spread it evenly, and place in the fridge for 4-6 hours (or overnight) until the mousse has fully set

Chocolate Ganache (optional):

• If making a ganache to decorate, make it when the cake has chilled and you are ready to serve. Heat the cream to just boiling and pour over the chocolate in a bowl. Rest for 5 minutes, then stir to form a smooth ganache. 

Decoration & Serving:

• Once the cake has set, carefully release the edges of the springform pan and lift them off. Peel off the parchment carefully, and smooth the edges of the cake as needed
• The ganache should be warm but not hot, and still a runny consistency. If it is not runny, microwave for 10-15 seconds until it is. Pour the ganache on top of the chilled cake, and use an offset spatula to spread it, allowing some of it to drip down the edges of the cake
• Decorate with chocolate shavings (optional), slice, serve and enjoy!
• Keep the cake cold, and don't leave it out for longer than 15-30 minutes (depending on room temperature)