Oh and while I was making the mousse the night before, I lost control of the whisking bowl and ended with whipped cream EVERYWHERE. So after changing my clothes, wiping counters, appliances, and floors I had to start again at about 10pm.
The decorating also turned out to be more time consuming than I had anticipated - I spent ages trying to get the sides to be perfectly smooth.
The mousse turned out amazingly rich and gorgeous, which I felt was enough to justify the work I had put in, however, there was no one around to eat the cake but me. Nevertheless, I would argue that it wasn't a complete waste of time as later I remade the very same mousse as filling in a chocolate cake, which was very well received.
In sum, always have a tight grip on your whipping bowl if you want to avoid late night laundry and cleaning.
I'm writing down the recipe as I used it but if you wanted a cake with greater height you could just as easily double it or use a pan with a smaller diameter (I used a 25cm round springform pan).
For the chocolate cake:
20g cocoa powder
1/4 tsp baking soda
1/2 tsp baking powder
1/8 tsp salt
Half an egg (yes, good luck separating)
1.5 tbsp vegetable oil
1/4 vanilla extract
Preheat oven to 350 degrees C. Stir all the dry ingredients (flour, cocoa powder, sugar, baking soda, baking powder and salt) in one bowl. Then add all the wet ingredients (egg, milk, oil and vanilla extract). Mix with electric mixer until combined.
Butter and lightly flour a round springform pan, pour in the batter, and bake until a toothpick inserted into the centre comes out clean. This should be about 20 minutes. Let it cool before releasing it from the pan.
For the chocolate mousse:
240g dark chocolate (70% cocoa solids)
4 large eggs
1 tsp vanilla extract
240ml heavy whipping cream (35% fat content)
Use a double boiler (or just a heatproof bowl over a saucepan filled with boiling water) to melt the chocolate and butter. Separate the eggs. Let the chocolate and butter mixture cool slightly then whisk in the egg yolks. Cover and refrigerate.
Whisk the eggs whites, using an electric whisk, until you have soft peaks. Slowly add 60g sugar, and continue whisking until you get stiff peaks and a glossy meringue (do not overdo this, your meringue will become dry).
Whisk the whipping cream, vanilla extract and the remaining 60g sugar until you get stiff peaks (again, do not carry on whisking once you reach this point as the cream can start to separate).
Using a plastic spatula, fold in the whisked egg whites to the chocolate mixture, a couple of spoons at first, then the rest. Then fold in the whipped cream. Refrigerate for at least a couple of hours.
A handful of strawberries
40g dark chocolate
Melt the chocolate using a double boiler as above, then dip the strawberries in chocolate and refrigerate briefly. In the meantime, spoon the (refrigerated) chocolate mousse onto the (cooled down) chocolate cake and smooth the edges. Sift cocoa powder on top to garnish and top it off with the chocolate strawberries.
Recipe sources: joy of baking, martha stewart