Dark Chocolate Mousse Cake

Instructions

PREPARE: Heat oven to 200°C / 400°F (180°C fan). Line the base of a 20 cm springform cake pan with baking paper and lightly grease the sides.
MAKE SPONGE: In a stand mixer, beat the eggs, caster sugar, vanilla sugar and salt on high for 5 minutes, until thick and pale. Drizzle in the oil during the final minute of mixing. Sift together the flour, cocoa and baking powder. Sift over the egg mixture and gently fold in with a spatula until just combined.
BAKE: Pour the cake batter into the prepared pan, smooth the top and tap the pan on the bench to release air bubbles. Bake for 10–12 minutes, until springy. Cool completely in the cake pan.
PREPARE TIN: When cool, remove the sponge, peel off the paper and place on a serving plate. Reattach the springform ring and line the inside with cake acetate.
MAKE MOUSSE: Soak gelatine in cold water for 5 minutes. Meanwhile, melt the chocolate in a heatproof bowl over a pot of simmering water. Set aside to cool slightly. In a small saucepan, heat 50 ml of the cream with the sugar until dissolved. Remove from the heat, squeeze out the gelatine and stir into the warm cream. Stir the cream mixture into the melted chocolate until smooth.
WHIP CREAM: Whip the remaining cream to stiff peaks. Stir a spoonful into the chocolate to loosen—it may look split but will smooth out. Gently fold in the rest of the cream.
FILL AND CHILL: Spoon mousse over the sponge, smooth the top and chill for at least 2 hours.
MAKE GANACHE: Place chocolate in a small bowl. Heat cream until just boiling, pour over the chocolate, add the oil and let sit for 5 minutes. Stir gently until smooth. Cool for a few minutes, then pour over the chilled mousse. Tilt to spread evenly. Chill 2–3 hours or overnight.
SERVE: Run a knife around the edge, remove the springform ring and peel off the acetate. Slice with a warm knife. Garnish with berries if you like.

Notes

If you’ve ever wondered how bakeries and food bloggers get such lovely smooth sides on these sort of cakes, it is because they use cake acetate. This is a glossy plastic that is easy to remove from the outside of the cake and leaves a smooth finish. You can buy this online or from good kitchenware shops. You can also use baking paper - I find that it tends to get a bit soggy and leaves wrinkles on the finished cake - it still tastes good though! Alternatively, run a thin sharp knife around the outside of the cake before unmoulding.

Gelatine can be very different in different countries. The sheets I use in Europe weigh 1.6-1.7g each and come in 'Gold Strength'. As a general rule, 1 of these sheets = 1/2 tsp of powdered gelatine.

If you are in a real hurry, this cake can be frozen after you have added the ganache. It will need to defrost for ½ an hour or so before serving, depending on the heat of your kitchen. In hot weather keep the cake cool until just before serving.